I must admit, I'm surprised at the reaction that phrase has generated - perhaps I could best elucidate with a brief list of examples upon which I could easily expand, as I'm sure you all would as well:
Asia
Styx
Foreigner
Journey
Giuffria
Bad English
Queensryche
Boston
Electric Light Orchestra
U2
...and the list goes on.
Hope that clarifies.
I think the confusion's coming from definition - these are all , in one way or another, rock bands (especially Queensryche). 'Mainstream pop' makes me think of the lowest common denominator crud that's clogging up the charts, although there has, of course, been plenty of 'prog flavoured pop' over the decades:
Beatles
Beach Boys
Kinks
Zombies
any other decent psych-pop band
10cc
Coldplay
etc.
Y'know, even Black Sabbath referred to themselves as 'heavy pop' early on in their career! But then, they did spend many years permanently stoned...