IRBs are not legally required, and most IRBs are excessively risk-averse (an IRB doesn't lose anything for denying a study, but a study with a bad outcome can give those on the IRB a bad reputation).

Suppose you are ethically allowed to do A (show ads), and are ethically allowed to do B (not show ads). Then you are ethically allowed to run an experiment in which the only thing you do is A or B.

You can definitely argue that showing ads (or certain ads) is ethically bad. You can also argue that there should be legal requirements for ethics reviews on all research (which is not currently the law).