This is the case in the UK (with 60 days instead of 90), and I think it's also the same in the US.

The US has something called OPT, which is essentially the same thing - you can work for any employer or for no employer, anywhere in the US. OPT lasts for a year, which is longer than the shortest time you could be authorized for under Canada’s program (8 months) but shorter than the longest time (three years).

There's a special talent visa that's general but it has much higher requirement than skilled worker visa.

It's completely inappropriate to repurposes the word "slavery" for H1B visa holders.

Sure, being tied to your current employer in a way that makes it hard leave is a terrible position to be in, but these folks are also well-compensated workers living comfortably in the richest nation on earth. They're free to travel anywhere in the US and are free to leave their job at any time.

Actual slaves don't spend weekends wine-tasting in Napa or cutting work early to catch a Giants game.