I take the position that Islamic terrorism present a real, albeit nascent, existential threat to Western society. I consider domestic right-wing terrorism a minor existential threat to current Western society. I do however, concede them to be a definite threat to Western governments which (rapidly) phase change to fascist-socialism.
The main distinguishing factor between "domestic" and "foreign" terrorism is the in-group out-group heuristics they prime.
Domestic right-wing terrorism is, for many Westerners, an in-group issue. These domestic groups' conservative nature ties them to a specific (albeit usually out-dated or blindly idealized) aspect of nationalism. So, while both domestic and Islamic radicals have government overthrow/change in mind, the former is somewhat grounded to the nation state's past while the later is uncertainly grounded to one of many possible futures.
Islamic overthrow/change presents an unknown future. It certainly could be that an Islamized West is a part of a happy pluralistic future. However, because Islamic (radical) groups are considered by many to be an out-group, motives are viewed as harder to understand, more uncertain, less trustworthy and potentially more harmful. Because of this, worst-case futures are primed. The cost of early intervention is relatively minimal (in the direct sense). Because of uncertain growth rates, late intervention is potentially devastating. Think Hitler in '32 vs. '40.
On the other hand, domestic terrorism is perceived as better understood. When in-group heuristics are primed, people tend to feel they have a better grasp on potential growth rates and ideological penetration depth. This makes them seem less scary. The actions of these groups tend to elicit crime/crazy categorization. They are a known aberration rather than a signaller of unknown existential competition.
Additionally, early false-positive intervention with an out-group has minimal in-group costs. Falsely rejecting an out-grouper has the potential loss of an unknown future benefit and zero immediate costs. A false-negative lack of intervention also has zero cost, but comes with an uncertain future cost. As long as you're surviving, we've been evolutionary selected for (varying degrees of) false-positive conservatism.
However, groups tend to do well with some risk-takers. That's why hetero-orthodoxes, like myself, consider tension between out-group orientation and in-group orientation necessary. Full isolation is an equally poor long-term strategy as full open-doors. Thus it is more than a little naive to bash in-group or out-group orientations. While ideological homogeneity may "feel" good, this is probably just a proximate reaction to the benefits of periodic freeloader purging (see Sigmund for some ground-work). Of course optimal in-group to out-group orientation ratios are environmentally determined. Is competitor group size growing? Increase via out-group orientation. Or, go small and strong and shoot for niche positioning.
Now all this changes based upon in-group out-group priming. Those on the far left are probably more likely to see potential domestic terrorists as an out-group and immigrants as an in-group. Conservatives are just the opposite. How you view each group probably tells you more about your own biases than it does about the rationality of either position. Neither position is inherently "bad". They are just different approaches to solve the in-group vs. out-group fitness riddle.
The far left tend toward out-group oriented universalism. That is to say, the far left tend to minimize ultra-nationalism in favour of inter-nationalism. In evolutionary sense, they're putting their bets on the returns offered by a larger adaptive group (ie. nation state vs. chiefdom). This is a good strategy. Larger groups tend to outcompete smaller groups. Out-group orientation also tends to minimize inter-group conflict frequency. This is one reason religion and universalist oriented religions have been so successful: they enable the formation of larger groups (see Norezayan for the causal arrow of religion->civilizaiton).
Similarly in-group oriented conservatives are oriented to minimize out-group swamping problems and favour in-group coherence. Haidt's moral foundations work gets at some of this. This is a good strategy. Coherent groups tend to outcompete incoherent groups. In-group orientation also tends to minimize freeloading costs.
The mathematical mean should be equally balanced between seeing domestic terrorists as an out-group and potential Islamic terrorists as an out-group. Where this mean is, is certainly debatable.
The US constitution certainly enshrines power to domestic purges of a technocratically detached & oppressive elite. As mentioned, domestic terrorism also has some ties to the average nation-stater's zone of reference, in the sense that it represents a plausible connection back-in-time rather than a potential connection forward-in-time. Pluralism and the marginalization of in-group oriented nationalism have certainly shifted traditional balances. My sense, is that progressive pluralism represents a super positioning facilitating a potential change to a higher adaptive group level which supersedes current nation state groupings. Its potential to minimize inter-national conflict is certainly real. Nonetheless, the strain on in-group coherence is also very real. Backlashes are endogenous not exogenous responses. However, all this depends on what level of immigration is considered fundamental to one's group. On this point, good people will disagree. Further more, they should disagree: group orientation heterogeneity is essential for adaptive group survival!