Some smaller consulting firms are honest and upstanding; I worked for no shady outfits during my years consulting in the US and Canada (about 20 years total between the two.) But I got the absolute creeps at some of the "offers" and onerous requirements from some of the larger firms south of the border.
For some strange reason, the large outfits think they're doing you a "favour" by sponsoring you, whereas the smaller firms recognize that you are committing to work through them for several years, too. It is a two-way street of benefit, but some firms don't see that. All they see is a lever they think they can use to increase their margins, which is a shame, because by doing so, they drive away the best of the talent, which isn't willing to put up with such nonsense.
The industry in North America has a special name for such firms: Body Shops.
They'll plop any BODY who claims to be able to do the job on site... as long as the margins are right.