Interesting! I've not seen them 'in real life' but from the photos on the net, they remind me of furniture moving pads - if you know what I'm talking about. If that's what they are like, then I think they would be really safe for the carpet.
Walmart carries them for $24 (out of stock online but local stores prob have them) vs $19,99 + 12.31 shipping if you order direct.
If you get them, let me know what you think