Mind boggling that someone could equate slavery via confiscation of passports to denying people immigration visas.
It is illegal to do this, enforcement is pretty aggressive.
As to denial of visas? At least the UAE offers these people a chance at a better life, Western countries give them nothing. A slight QOL improvement is much better than none.
To get a low-skilled work visa in Dubai you have to sit through an hour long lecture explaining these rights, including what to do if your employer takes your passport.
You are advised to not give your passport to your employer except to have your visa stamped, and then immediately demand it back.
This country is modernizing very rapidly, things that were true 5 years ago are often not true anymore.