Daily Roku user here. I'm not sure what you mean about Amazon and the fastforward button.

Apparently leading up to Amazon prime video inflicting ads where no ads had gone before, they put out a requirement to streaming device manufacturers that said devices needed to support the ability of the source to disable certain buttons. Couldn't have people fast-forwarding through the precious ads, now could they?

So when they just stop your show to try to sell insurance or whatever, and you try to ffwd through that noise, you get a little caption that informs you that "fast forward is not available during ads".