Ok correct me if I'm wrong, but RSUs are given to you as a part of your income, right? You don't buy them? That would make sense then to tax them as compensation. If the company gave you other non-cash items as a form of compensation, you would have to pay tax on that too. If the company rents an apartment for you, the value of the rent would be considered income.

That is a lot different than buying a stock at $10, having it go up to $100, and you now have to pay taxes on the gain (which may be temporary, the day after you pay taxes it could go back to $10).