I think the issue with moderating is that people are allowed to hold and express views that some on here (myself included) would consider to come from an essentially racist worldview, provided the comments are not explicitly racist in themselves.
It's up to MN how they moderate but it's always been pretty much a 'free speech within the bounds of the law' approach and I think that's a good thing.
They can obviously look at posting patterns - a hypothetical poster who continually derails threads for example, or obsessively posts about immigration could be viewed as 'not in the spirit' or not a 'genuine' poster, but OTOH immigration is a big political topic and some will have a particular interest in that as much as any other area.
Derailing is the thing that bugs me most but the way threads seem to work is that you can crowbar basically whatever you want in because conversations may be wide-ranging and posters are entitled to express their views even if they're not strictly relevant to the thread title. And again I think that's fair enough as a moderation policy, but it does make a lot of threads very contentious and meandering. It probably also drives traffic as the urge to push back at annoying posters is a strong one.
My own views on immigration etc (not the topic of the thread obviously) tend to the conservative and I also worry about extremist views, misogynistic cultural norms etc, while not disputing we have plenty of home-grown issues of our own. But I think the sheer number of threads about this at the moment and the dodgy statistics and inflammatory rhetoric is getting quite out of hand and must make anyone not white British question how welcome they really are. It's a shit situation and I think it's being driven by a combination of our weak immigration policies and enforcement and the professional racists and provocateurs getting every last bit of mileage out of that. A very toxic mix.