The trouble with volunteer GMs in that in community-run games, 80% of people who volunteer to be a GM do so with the intent of abusing GM power to give themselves and their friends free stuff, instant 50s, whatever.
So you then need trusted people to police the GMs.
This. Oh soooo much this. The biggest problems with volunteer GMs is oversight and quality control. A long time ago Vivendi, I think it was, tried this with Everquest. You had GMs abusing their power so much it caused so many PR problems that they finally had to get rid of them because of the backlash. You had stories of people asking for help from a GM and when the GM would show up they might help with the problem but then have something like this happen
Player: hey thanks for the help
GM: No prob...hey that's a really nice sword
Player: Thanks. It took me weeks of raiding before I got it to drop
GM: Yeah, its hard to get. My friend wants one. Give it to me
Player: well he can raid for his own, like I said it took me weeks to..
GM: give it to me or I ban your account
Or there the was rampant
GMs friend: I "accidentally" deleted my starter armor, can I get a new one?
GM: I can't create the item,, but I can give you what its worth in gold... oops accidentally added ten zeros to that. My bad
Granted these are extreme but all it takes is for one or two volunteer GMs to ruin it for everyone, not to mention ruin the rep of the company running the game. And then there's quality control, GMs who don't know the game as well as they claim, or even givig customers the same level of treatment, or same amount of knowledge. That is done by having all GMs be employees.
There's very little consequence for a volunteer GM if they go bad. The damage they can do before they get caught is staggering. And a company would be foolish to trust their game and reputation on, even if well meaning, people who they can't have effective oversight on. ie having the GMs be in the same place as their supervisors.