FDA labeling is generally fine, but would we force an artist doing a large edible art installation to slap nutrition labels on it?
Personally, I don't really see what the big deal is. Consumers shouldn't be naive enough to buy something just because a celebrity likes it, but then again I've never understood the appeal of celebrities in the first place.
Celebrity isn't so easily defined. What if Bruce Schneier, who you could consider a "celebrity" of sorts in the security community, started (subtly) promoting Signal Messenger in his tweets and blog posts. Wouldn't you want to know if he was being paid to do that?
This is essentially what is happening when PewDiePie promoted a video game.
In many cases I've heard them advocate as if they've used the product or service personally and got a good deal and there's never a "paid ad" or "promoted content" disclaimer (or maybe it runs at 3 AM?).
As for the comparison with FDA labels--you can't look at just the fact that its "compelled speech" in the abstract. The back of a product box and a celebrity's instragram are very different venues in terms of expression.