I would go along with the DJ excuse, EXCEPT that was happening to me many years ago, back before DJs, when I was playing organ.
I don't think they MEAN to be rude; they are just a little self-absorbed and/or drunk.
There have been times when I have had to just stop in the middle of song to keep them from climbing all over my gear. Of course there WAS a time when I didn't mind some of them climbing all over my gear.
On a similar "note", I had a guy come up last week when I was right in the middle of a song and ask "Can I sing?" Between lines, I said "sure, go ahead", and I went back to singing the song. He looked at me like I was the stupid one, but he went back to the table.
I remember another occasion a few years ago when a woman came up and started talking to me. I turned to her and said "sorry I can't stop this thing in the middle of a song". THAT she believed!
I believe that these days, it's more the Karaoke thing that has people confused. They are used to the Karaoke guy singing when nobody else has their name in, and they think it's fine to step up and volunteer to finish the song for him.
I haven't yet been in a situation that made me want to quit what I do and start getting up at daylight and sitting at a desk from 8-5!
DonM