i've been having this conversation a lot recently. 'being professional' is not just down to the artist. an artist is always caught in the line of fire because on the one hand they are there to entertain and 'the show must go on', on the other hand, they damage their reputation if the sound is wrong (or non-existent).
we the paying public should be complaining to the venues and promoters when sound is bad, not blaming the artist.
As much as you can have technical riders signed by the promoters before the gig specifying a certain set-up, and do a sound checks, event organisers rarely have qualified, capable sound engineers on hand to make sure everything goes smoothly. even when they do, things can still go wrong.
An artist risks looking like a prima donna if he storms off stage because his mic isn't working properly. He also risks losing fee if he doesn't perform. Don't forget, an artist can't always hear that the sound is bad from on stage - he's not necessarily hearing what you're hearing as he's behind the speakers. That's why you should have the sound engineers behind the crowd facing the stage.
Next time the sound is bad at a gig, demand your money back from the promoter or the venue, don't blame the artist.
