No, it's not a bias towards mages, it's a bias towards anarchists, warmongers, vigilante, rebels, and radical/extremists.
It's not just Riot, a great deal of writers possess this bias, because it's what sells, because many among the audience members are shallow and egoistical edge-lords who don't know how to do things properly and their only solution that they reach for FIRST BEFORE ANYTHING ELSE when it comes to anything remotely complicated is "destruction". And so writers cater to this demand.
It sucks, but what can you do?
.....
This isn't to say Riot never managed to write good stories that make good use of a conviction for something grand and a well-purposed use of strength, I mean we at least have Yorick to show for it (and is it a coincidence that the champ is so unpopular and so low on the priority list he is virtually forgotten from skin releases? Who knows), but many do fall into the really "meh" category of just shamelessly catering to edge-lords.