> Could Amazon disallow that if they wanted?

It would make sense for Amazon to raise the price rather than disallowing it (specifically, because enforcing a prohibition on activity would cost Amazon money, but setting the price to be at least marginally more expensive than Amazon's expected minimal-effort realizable gain from using the GPU instances themselves would make Amazon money.)

No they could not, it's just OpenCL. Also: Why would they? They still get paid and the GPU is dedicated on those instances.

Why wouldn't Amazon just then dedicated "all" of their free resources towards mining them, and keep whatever profits other are making?..