I think it would be nice if there were clearer instructions. Plus, I'm hesitant as I don't know if texting 'STOP' to a +1 (US) number from the UK.
I've just looked up the cost, it would be about £0.20 to send it.
I did tweet them about it a while ago, but I din't think it really made sense.
https://twitter.com/Martin_Adams/status/702074369296769024