Speaking for myself, the reason I sometimes discuss or debate domestic American political issues (like reproductive rights, gun laws, etc.) on RF is twofold:
• First, I see those as issues that affect millions of people, much like, say, the issue of religious freedom in parts of the Middle East. It usually wouldn't bother me if an outsider discussed the latter from a place of genuine thoughtfulness and concern rather than moralizing and self-aggrandizing or self-serving lecturing. If an issue deeply affects millions of people, regardless of where they're from or where they live, I think it's understandable that some outsiders may become interested or invested to one extent or another in that issue.
Of course, one could ask, "Why be interested in this domestic issue of X country/countries and not the domestic issues of other countries out there?" I think the answer to that will almost always be subjective and/or dependent on personal factors, such as whether one has direct personal connections like family or friends in X country. People don't tend to be consistently and equally interested in issues from all other parts of the world, and I don't think anyone could even be so—their emotional bandwidth and capacity for absorbing information would simply hit the limit. For example, I admit that I have read far less about, say, internal strife in Colombia or cartel wars in Mexico than I have gun laws or abortion laws in the US. This ties into my second point:
• I don't know anyone in Colombia, but I have a lot of friends from the US or who live in the US. This doesn't mean that the well-being and rights of Colombians matter any less or any more than those of Americans, but from a strictly personal viewpoint, I have more reasons to care about American abortion or gun laws than guerrilla wars in Colombia or cartel wars in Mexico. Not that I can change anything in either case, but I will naturally be concerned when a friend leaves a state for another because of a mass shooting at a mall near where she lives, or because abortion has been so severely restricted as to practically be almost banned in her state.
I do get your point about presumptuousness and intrusion, though. I think I can relate to it when I see people make condescending, misinformed, and sometimes downright racist claims about what the culture or people are like in my part of the world, without showing much or any nuanced understanding or having any direct experience with the places and people they so readily generalize about. I have seen such rhetoric about the US and Americans, including stereotypes about how Americans in general, rather than a subset thereof, are supposedly obsessed with guns, don't know what's best for them, don't know anything about the world, etc. Such generalizations strike me as ironically an example of being ignorant about the world, in addition to being prejudiced against an entire country and its people instead of having more focused and grounded criticism.