Is it polite to pass gas on a crowded bus?

Yes, farting in someones face is especially rude. ;+

But it's OK to scratch your nuts in public?

I didn’t say that. :) 
There are different ways to go about it. 

Sounds like a Trump backpedal. I really am laughing out loud.

I think scratching one’s nuts is about the same on the politeness scale as pulling out a wedgie.
Gotta get it done, but no one wants to see it. ;-)

Farting is an art form , not an offence. ;+

Like some things people will claim as “art”, it can be offensive. 

I suggest you endure being “cropdusted” in an elevator, ...Art? 

I just read on the internet on one of those "what airline, Burger King, etc. employees" know happens.

If an airline steward or stewardess times a "fart" perfectly while he or she is in the vicinity of a particularly annoying passenger, that is supposedly referred to as "Crop dusting" in airline "code" language.

Did you read that article also?

I am not aware of the origins of this use of the expression, but I have had a few laughs employing it.

Hope it was well-received here. This post was edited by Don Barzini at May 8, 2018 4:27 PM MDT

Looks like they cut one for him too. ;++

Yup!
:)