This a letter anyone can sign. As someone pointed out Grimes is one of the signatories. You can sign it yourself.
Hinton, Bengio, Norvig and Russell are most definitely not getting prestige from signing it. The letter itself is getting prestige from them having signed it.
Academics get paid (and compete hardcore) for creating status and prominence for themselves and their affiliations. Suddenly 'signatory on XYZ open letter' is an attention source and status symbol. Not saying this is absolutely the case, but academics putting their name on something surrounded by hype isn't the ethical check you make it out to be.