Because i don't know if supporting the artist its consider as a donation, as in which case i think its legally fine for the authors to get the money as they are not selling and people are donating its not a consider(if remember well) a real transaction and its just encouraging them to keep drawing, you are not paying for the comic, not really, the comic is free, you are only giving money to say to artist"i like what you do, i want you to do more of this"
In that case they earn money from the comic from creating it? yeah, but if it is considered a donation, they are not getting paid, we are giving them the money(in this context i just talk of tapastic support, not ad revenue or premium) from something free, i suppose saying it is considered as a donation it would be fine.
On other hand yes they are earn it from an intellectual propriety so i think it is in a pretty grey spot(only the support program, obviously in Ad Revue and Premium it would illegal) or at least i personally dont have this absolutely clear
Or if it is @VeloursRose said they might fall in parody category in which case parodies does not get in problems by copyright laws.
To be quiet fair i am not a lawyer, but in my understanding, it would be fine if it is consider a donation if someone know if this is ok, it would cure my curiosity