The issue with a charm spell is that it is so blatant. How could one be charmed when a person obviously casts it right at them RIGHT in front of them? Noticing the spell being cast would immediately piss most people off, as the caster is clearly altering something about them. Its kind of a push. You cast a spell to charm them, but them seeing it should piss them off, countering the effects of the spell.
I agree, charm spells should be more difficult to cast, considering illusionists could run around charming everything in sight without a problem.
if charm is illegal than debates would be illegal because they are trying to use something to change your ways of thinking, which is kinda part of the legal process in itself
Charming is the equivalent of a temporary magical lobotomy. Debates involve two parties discussing things in an orderly manner, not smashing pins into their skull to make them agree with you.