Yeah that one gets me too. Here's a funny one though, I wrote in a message yesterday "They've been putting on musicals, how good they are depends on who plays the leads." Sounds fine, right? Only that's not what I was originally going to write, I was going to write "How good they are depends on who's playing the leads", but then I realised that it should be "are", so I went to change it to "How good they are depends on who are playing the leads", but that sounds totally off!
I haven't figured it out yet. I realised a while ago something similar with "I've". You can say "I have got it" and hence "I've got it", but although you can also say "I have it", you can't say "I've it".
I think contractions do actually have slightly different meanings from the words their contracted from. My dad was taught that "none" is a contraction of "not one" and that it should therefore be treated as a singular. That's fair enough, in the case of "None of the ships was sunk", but it also means "not any", or it wouldn't appear in sentences like "None of the beer made it into the glass". Also, if anyone's applying grammar rules strictly, that should also mean that it's n'one, and that plurals of acronyms are similarly apostrophised, which the strictest in grammar are the least likely to do.
But yeah, I don't think there's any exception with "is". It's used on plurals in pretty poor speech and that's annoyingly widespread when it's contracted. You can actually say "there're" but nobody bloody does it!
