Well, at first sight I would react the same way : I don't remember how writing to a Flash memory works (and BTW there are plenty of different species these days), but I don't think there was any sort of handshaking that would put you in a bad state if ever you were to slow as a host.
If you try to write after the card dropped access, you'd be writing to the bit bucket. I suppose it's possible to write to a class 10 card with a controller that can write at speeds so high that the wave function of the electrons damages the Strong Force of the copper in the card, but I'll leave that to the Quantum Chromodynamics guys. (And that would turn a class 4 card into Space Warp Cheese.) I have RAM rated at ms speeds, being written to by CPUs running at GHz speeds, with no problem at all (other than the huge number of wait states generated).
No, the "Class 10 cards are too fast" is the lie of the day. I wouldn't even pull that on an office manager, let alone people like some who post here and actually know how semiconductors work.
I can make a couple of wild guesses, though. Sammy may flip modes if you use any card but theirs, and rewrite everything thousands of times. That'll eat cards faster than a starving man eating steak. Or they may use an SSD model to distribute written data if the card is theirs.
Or they may have just made a crappy phone and they're trying anything to kill the bad publicity. (Which isn't working so well, since there's been so much of it the past few months - every phone they come out with seems to have more bugs than features.)
Remember the company that came out with a personal computer that pretty much defined the term? That defined it so well that if your computer wasn't compatible with theirs you might as well go into the pencil-selling business? The company that pretty much defined the top operating system in the world? Then got so big that it decided that customers were just expenses to be eliminated? When's the last time they came out with a new PC? (How do you translate "britches" into Korean?)