120 hours a week. So...17 hours a day, 7 days a week eh?
Show me where you get these numbers please.
Honestly, I've worked with many temps - and they're rarely given OT. If they do, it is usually when they have an extended contract, and the employer specifically asks them if they want to work an extra shift or stay extra hours.
Capping it at 30 hours would be pointless. Ok, so you give more people jobs working less hours....so that they have to go get other jobs to make up the extra hours they could be working to make a full time income. Now instead of getting a steady 5 day work week of 8 hour shifts, they're working split shifts among varied days...wearing them out and preventing them from spending time with family. I don't see that as being any better.
Besides that - it isn't the temp agency's job to make sure there are "more jobs available" they're in the business of providing employers with workers, and providing workers with a paycheck.
If you're going to do that..might as well go all the way and put a cap on regular employess to work 30 hours instead of 40. I don't think that'd make anybody happy.