I have read that language, saying that discount does not apply to taxes and fees, on the back (or on the online terms) of several UA certs I have redeemed, though I'm not sure if any of them were DOLLARUA certs.

If the term is not on the cert, I don't know whether it's lawful for UA to add the term. It might depend on whether there's a term on the cert saying that UA can change the terms or revoke the offer at any time.

I think it would be unethical, but I think that at UA and most other airlines these days, in a conflict of ethics vs. $$, the $$ will always win.