I agree that as a general rule stuff like this should be reserved for powers. That is any kind of move that PC is going to use often, they should need a power for. However there are those moments when it just makes a lot of sense for someone to try something like that and there is no appropriate power available.
That is one of the things the stunt system is for. Just make the PC grab the target and then expend another standard action to toss the monster back a square and knock it prone, using another STR vs FORT roll to succeed. Allowing some small amount of damage to result is OK as well. I think it would also be appropriate to rule that throwing provokes OA.
Mechanically my argument goes like this: It requires 2 standard actions, with 2 attack rolls required in order to succeed. This makes it inferior in general to any power, including the basic bull rush action. On the other hand it does knock the target prone and it does do SOME damage. Prone is NOT that big a deal. Yes, it grants CA until the target stands up, and effectively immobilizes them for a round, but in most situations the latter factor is not all that big a deal. The CA is pretty good, but there are plenty of ways to get CA and most of them are cheaper than expending 2 standard actions.
It is unlikely that PCs are going to start doing this all the time for a number of reasons. Really in 99% of situations an attack form that requires 2 standard actions and provokes OA to do say 1d6 damage, push, and knock prone is pretty weak compared to using a bull rush plus an at-will to do the same pushback. Second it will quickly become obsolete because you can't get enhancement bonuses to unarmed attacks. So by paragon tier it will already on average be subject to a -3 to-hit effectively for each attack roll, plus by then PCs will certainly have powers that do similar things more effectively and without the lack of enhancement bonus.
I know a LOT of DMs these days seem to be of the opinion that these kinds of things just should be disallowed as 'unbalancing', but if all anyone can do are the stock actions then why not just play computer games? The whole point of a human moderated game is flexibility. The last thing any good DM should be is inflexible.