They shift to reserved instances, not spot instances. It'd be hard for a service provider to shift their customers instances to spot instances unless the custo

They shift to reserved instances, not spot instances. It'd be hard for a service provider to shift their customers instances to spot instances unless the customer could tolerate the spot instances being shut down on short notice, and if they can, that customer may as well just use the spot instances themselves. Worst case, this will increase usage of reserve instances and reduce on-demand usage, but AWS priced them accordingly, so they don't care