The long and short of it is that there are different levels of scrutiny for different kinds of speech. First, "commercial speech" is narrowly defined. It is speech that does "nothing more than propose a commercial transaction." Second, even commercial speech receives First Amendment protection, just less protection. Instead of strict scrutiny, the Court applies an intermediate standard: the law must advance an important government interest by means that are substantially related to that interest.
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.
I think the murkiness of cases like this is that it's not clear that the speech is purely commercial. As someone mentioned elsewhere, it seems somewhat analogous to product placements in rap songs. There's clearly some commercial activity going on, but the root activity is also artistic—rappers would rap even if they didn't get paid for it, and celebrities would maintain social accounts even if they couldn't do product placements there.
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.
There is hubris in thinking you are immune to marketing, and if anything it makes you more vulnerable to it.
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.
Turn on many pop or hip-hop stations in the US in the morning and you'll hear the actual morning show host endorse all sorts of products that sound like a scam: discount cosmetic surgery, car rims for rent, accident attorneys and more.
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.