It's not a matter of guts, it's a matter of what makes them the most money, and/or what's required of them as a common carrier. If airlines could make more money enforcing a dress code, and assuming they're allowed to, then they would do so. That they don't is a pretty clear sign either (A) they would lose money by doing so, or (B) they aren't allowed to.

Nightclubs/discotheques are entertainment venues, not transportation, nor do they have any common carrier obligations, nor is there a risk of someone showing up with prepaid tickets and then having to deal with IDB because they're inadequately dressed. You're comparing apples and ball bearings.