[Gameplay] Gift of the toadstool applies Liandry/Rylai/Luden's passive while Thornmail doesn't

This is bugging me quite hard actually.

Thornmail and other items that dealt magic damage passively were made incompatible with Liandry/Rylai when they were first introduced, in order to avoid the stupid abuse case where you'd go thornmail+liandry+rylai and wait until the enemy dropped dead from their own damage. This was a reasonnable decision.

It seems however, that this does not apply to gromp's passive - Gift of the Toadstool - as it does proc "on magic damage dealt" items. This seems a bit weird, especially when this "mechanic" can only be accessed by junglers - or at least to people that have smite. There is very little counterplay possible for an adc to the build liandry/rylai + smite on gromp, as a single auto on the bearer can result in the loss of 20% of your current HP.