Mostly the whole exploitation part, which often goes too far and can be very humiliating or outright dangerous for participants. Some scrapped videos supposedly amounted to torture. Then there’s the rigged giveaways and the fact that the dude just unnerves a lot of people because he, as this post demonstrates, doesn’t smile with his eyes.
There were also allegations that one of his colleagues was a sex offender but as far as I know everyone including the purported victim ended up denying it, so I don’t know how much people care about that still.
Sending a simple transaction like this costs a couple cents though, which they could in theory bill to the developer as well. Setting the threshold at 100 is probably more to accrue additional interest on Steams bank accounts.