Frankly, I doubt that they'd steal the money on purpose. It doesn't make sense as a "business" decision. First of all they're presumably going to have to pay their customers back which is going to put them on, at best, an even footing in that they'll have their $15 from everyone for the month, but no more. At worst, they'll also be obliged (by pressure from the fans or the PR department or whatever) to pay the players' wages for the month in which case they'll actually lose money.
Furthermore they're going to have a large percentage of their player-base canceling their subscriptions now, so they've weakened, if not crippled, their business in the long term. Add to this that Mythic is now going to be considered untrustworthy in the wider MMO community, so it may be that fewer people will play their games in the future as well.
And what if somebody tries to sue them? Someone's going to have to pay for the lawyer fees I would imagine. And so on and so forth.
Really they gain absolutely nothing from this, except perhaps a couple weeks of drastically increased capital which will be lost again at the end of that time span (however long it takes for them to give the money back). As a trade-off (albeit a poor one) they lose many of their subscribers and have a huge knock against them in the community, so have lost potential new players as well.
If this was an intentional act then I would guess that it was on the part of the company they hired for billing (but again, for what purpose?) or was committed by an employee of Mythic, or maybe several (so...what? They can be fired?) rather than by the whole company. Maybe it was a Blizzard saboteur?
