It may surprise you, but streamers are selling a product. That product is you, the viewer, to companies who want to sell stuff to you. This selling of viewers is done via sponsorships. Sponsorships aren't inherently bad and are even often beneficial for the viewer, oftentimes they'll give you a discount for their products using the streamer's affiliate code.

Trainwrecks has a sponsorship with Stake, a gambling website which is illegal to gamble at (in the United States). Trainswrecks is circumventing this rule by gambling in Canada since Canada has more lax online gambling laws. Presumably, Trainswrecks is getting a kick-back from Stake from sending his viewers to the website.

What's the problem with it? Well, it's because the product that Stake is offering is almost guaranteed to lose you money with no return in value and unlike many other products has the potential to get you addicted (you aren't likely to buy a Logitech product every day). Trainwrecks is willing to risk you getting addicted in exchange for his own personal gain.