I know this has been a year, but were you perchance playing at NAMM? I'm playing there next month, and they have this same arbitrary 40W restriction.
It only makes sense if it's for available power on-stage and they don't wanna overload their circuit, otherwise it's to try restrict volume making it nonsensical as I could stroll in with a 40W head and a 4x12 and be extremely loud.