It is only because of the UI that East knows what West has. West's 5
bid does not show "only a 2 card support" from the bidding: it looks as though West has got six or seven card suit that West deemed unsuitable for a pre-empt.
How does East know his 2
opening was the Multi? From partner's explanation, of course. So he must make every effort not to take any advantage from the UI available to him, and this he has failed to do.
You ask where the infraction is. East has chosen amongst LAs one suggested by the UI: pass over 5
is certainly an LA. I am quite sure that if West had said that East's 2
was Multi that quite a few Easts would now pass.