As has been said before, the assumption is made due to the fact that 90% of people who ask about "downloading" on this forum are doing it because they've heard from a friend that mIRC is the next wonderful file sharing program.
I think in the few years I've been here, I've probably seen 5-6 threads where people have replied saying "I wasn't trying to share illegal files", compared to the god-knows-how-many threads where there is a) no reply (I wonder what THEY were doing), or b) "Ok, sorry".
If people would be specific in their questions, time would be saved. Most of the time though they are not specific because a lot of the people don't even realise mIRC is anything but a file sharing program, so they just assume people know what they are talking about. It's the age old problem that people in general are lazy and cannot be bothered to read anything, even the front page of the site!