These things should be decided democratically, not arbitrarily by the gate staff/duty manager/whoever.
They should ask any passenger likely to be offended by the tee shirt to press their call button. If more than half the call buttons are pressed, the SLF doesn't board. Sounds quite fair to me.
What would they do if he'd rocked up wearing a "Vote for Hilary" tee shirt. Or even "Vote for Obama". They'd probably let him on, because it's a positive message. Unless the gate staff don't know the difference between Obama and Osama.
Worse still, suppose he wore an Australia Day tee shirt, and they let him on, then he changed into the anti-Bush tee-shirt in the bathroom overhead Uluru en-route to the UK, would they chuck him out?
For Pete's sake...