A smart plug must extract a small amount of power from the line to run itself--and that means a power supply that very well might care. There's little reason to make the power supply universal as the plug is part of the unit and the plug specifies the voltage. You very much do not want plugs meant for different voltages to be interoperable! (20+ years ago I had multiple run-ins with this. China uses 240V and one of the two plug types is the same as US 120V. My FIL would try to plug US stuff in without using the transformer and of course it would promptly self-destruct. By now most things one might be traveling with are universal voltage but back then few were.)

Not only are US 120V plugs different than 240V plugs, but the 120V/20A plugs are a superset of standard 120V/15A plugs--a 15A plug will fit a 20A receptacle but not the other way around. I do not believe code permits plugs for devices beyond this range--the really heavy stuff is always hardwired. I believe this is to prevent morons from trying to power their house from a generator with a suicide plug--forcing people to use a proper transfer switch that can't backfeed. I suspect this will change in the future due to how dirty small ICE engines are--as it stands things like electric mowers aren't very good because they can't get enough power to be good. Likewise, pressure washers--the high power versions are always ICE. Where I grew up you needed at least 4hp in a lawn mower--and you can't draw 4hp from a normal wall plug.