They just want to be able to deny a claim when their POS battery craps out.
As long as you have a decent battery maintainer/tender, you can keep it on a battery year round, provided the battery isn't a POS. After all, that's the entire point for getting one!
FYI: True Amalgamated literally told me, "... our isolator is set up and manufactured to run on a tender."
I think you're right on that one @Scoop I got the same battery with the same warning. You know they didn't do anything fancy with the battery type and construction. It gets subjected to 14 volts (or there'bouts) when motoring around and that doesn't appear to hurt anything.
Like others, I installed a water-tight charging socket in a convenient spot (in the plastic of my bumper. When charging, I take the key out of the ignition and put it in the cup holder . . . otherwise my wife or I will surely try to stretch the charging cord.