Well, either that, or they know that more people would use those microwaves than buy those box meals... they'd really lose $$$. Not unless they implement "pay" microwaves and charge $1.00 a minute...They're fine with me sharing a microwave with my coworkers.
It's probably more that some state's health departments don't allow it & since Amtrak rotates cars around, they can't guarantee that the cars with microwaves won't go thru states where it's not allowed.
peter