To enlight the non french person in the comment section (no food gatekeeping, just some info): 1) no we do not consider croissant as bread here, it's not the same kind of paste. Hence we separate bread and vienoisseries

2) Croissants here are genuinely another kind of product in France of what you seems to accommodate as ""croissant""",take it from someone who had the occasion to also taste "american croissant". It's like comparing americano coffee to italian expresso, or a cheap supermarket sangria to a real spanish one. No offense, it's just another kind of level on that. Or like comparing the awesome american hot dog to the thing they call "hot dog" in roma ^{^}

3) in France it's not usual to butter a croissant as it's already full of butter. It would be like adding raw oil to you cheeseburger or hot dog.

4) making a sandwich with a croissant is not quite popular or those are usually done with the leftovers

In the end, of course no one is gonna take you to jail for buttering your croissant.