I'm in the US and this sounds... shady but not abnormal.
The company is required by the IRS to do withholding on supplemental income (which RSUs qualify as). There are usually three ways to do this: 1) the employee puts up the cash before vesting day, 2) the company keeps some number of the vested shares and then sends a check to the IRS from their own bank account, or 3) the company sells some number of the vested shares on vesting day and sends the proceeds to the IRS.
I've seen #1 and #3 personally as options when I've had RSU grants. I always chose #3, but I could have also chosen #1 and kept enough cash in my linked brokerage account to cover the tax withholding.
> When the company IPOs, your vested shares would immediately vest into actual shares. At that point, you would be taxed [...] Your RSUs are now shares and when that conversion occurred on IPO date, you would have been taxed. You own no more tax until you sell your shares.
No, that's not how it necessarily works. Companies can hold off on delivering the (vested) RSUs until you are actually able to sell them. That's a good thing, because no one wants to have to pay tax on their vest date if their shares aren't liquid.