Part of an effort to incentivize colony world variety, yeah; most items have requirements or interactions with planetary conditions.
Pollution doesn't require habitable - consider corrosives, radiation, etc. Good point re: bombardments, though; let me remove the "habitable" requirement there. Hmm. On the other hand, what this does is instead of disincentivizing industry on habitable worlds, it more incentivizes (somewhat) industry on a world by itself. So, actually, let me make a note to check tomorrow; it seems offhand that requiring habitable for pollution is probably the right way to go.
Re: whether it's heavy industry or a nanoforge, I'm not sure it actually matters all that much which one causes it, since a nanoforge of some sort is almost required. But I suppose this leaves the option of having a safer, low-tier heavy industry, not that it's going to generally be a good option.
Heavy Industry is a great filler for boosting production limits. If I am avoiding Pather cells (I did before I knew about the Pather bug), I do not use forges on additional Heavy Industries. Anytime I have spare industry slots, I build a Heavy Industry. Also, heavy industry produces machines, and it is nice to have a regenerating stack of machines in colony resources. (Waystation only handles supplies, fuel, and crew if demand is met.)
In my last game, I had three or four Heavy Industries, but only one or two of them had forges. The rest were there to boost production so I can build a bunch of ships (or a capital) and/or weapons in a month or less.
It would be annoying if Heavy Industry alone (no forge) caused pollution. Might as well have eco-terrorist Pathers form permanent cells if Heavy Industry alone is that destructive to the planet. Would also make the Ludds (both of them) total hypocrites, since the Church has Industry and corrupted forge on Asher.
P.S. Allowing pollution on non-habitables means bombing such planets is no longer a clean and easy way to destroy and steal a non-habitable enemy world. (For habitables, I would want to decivilize it with raids then abandon it to get a clean planet to seize instead of sat bombing it. Of course, with that abandon bug fixed, probably just bomb habitables too since Pollution is less punishing than Decivilized.)