Of course the imperialist nature of the "occupation" is never properly explained, mostly because it is fantasy and rhetoric. NI is a tax sink to the UK, if they could ditch it tomorrow they would, unlucky for them they can't.
This is kind of the nub of the question isn't it? Why did (does) the British State care so much about NI?
But simply to say, "it wasn't making them money therefore it can't be imperialism" doesn't do the job. There are and always have been, a whole bundle of political, economic and ideological bases to imperialist ventures and my God NI is mired in them for the UK state. And of course many of these are historical ones that no longer really obtain but have dug the British into certain positions which are hard to get out of. And most of the senior military actors in the late 60s were men who had been in the army almost since the original partition, this was actually personal to them.
45 years ago when the British state leapt into action in NI there were a whole range of reasons for it to obsess about NI even though it was - materially - surely not that important.
1) There was a narrative of imperial decline - based on historical fact, the end of empire the shrinking of British influence. Obvious but easily forgotten now, for the British ruling class this - as individuals - was a profoundly shameful and painful experience, a political and economic degradation in the eyes of the world. There were important elements of the British state and army that were looking to draw a line somewhere. Moreover most of the major global liberation struggles had taken the form of national-liberation struggles, post-colonial ones, and for Britain to fight and win one of these (a la Malaya) was really important.
2) There was a narrative (just beginning) of 'social' decline, mainly based on a change in the balance of economic power in favour of the UK working classes through TU activity. The barbarians were at the gate. There was the rise of 'hooliganism' of all stripes - and in places like Derry this was seen as becoming politicised, separately from anything the PIRA or SF were up to - the whole DYH categorisation (I don't know much about this but I find the name interesting). Again, there was a line needing to be drawn. The 70s in particular was a time of pretty open class war in the UK and the vision of a more-or-less openly leftist armed rebellion in working class ghettos within the borders of the UK was one that was always going to need to be utterly stamped out.
3) 45 years ago anti-catholicism was still a defining feature of elite ideology in Britain - I remember when Pope John Paul II was selected and suddenly there was a glasnost on that one because he was anti-Soviet and Polish, but that came much later.
4) the collapse of traditional industry and its complete replacement by a finance and service economy was all incomprehensible in 1970. We were still being exhorted to Buy British and there was public scandal about "our" companies being outperformed by foreign ones - so NI's industrial base, hopelessly old-fashioned as it now looks, was still a prize to fight for. There was plenty of statist thinking on the right in those days, saving "our" industries, anti-EEC etc.
I could rattle on, my point is that the fact that Britain wasn't making a profit on the NI account doesn't mean this wasn't an imperialist adventure carried out for imperialist reasons. This whole "they'd (we'd) have got rid of it if they (we) could but no one else wanted it" line is one that the UK elite, especially the liberal-media part of it pushed repeatedly through the whole conflict but it's crap (and it's essentially racist too, you know what the Irish are like rolleyes/snigger).
The UK state never sought any policy of trying to "rid itself" of NI, ever. Important elements of it obsessed about keeping it in the UK. Irrational? Yes - to a lefty (or even, embarrassingly, to a social-democrat or liberal which is why the usually very slick and confident UK media struggled so hard with this issue), but ideologically coherent and politically utterly necessary from an imperialist pov. despite the fact that the whole campaign was a massive PR disaster abroad. My experience was that when I was abroad, the whole war was seen as part of a British problem, not an Irish one.