Most bakeries offer the two types though: "au beurre" and "sans beurre". The later don’t necessarily needs to be made with cheap margarine, a good vegetal oil (or a combinaison) suits very well. The flavor is different and I prefer those than the butter ones. Freshness is a must obviously as peer said.
You're right, and I was simplifying. The problem isn't margarine per se, it's cheap margarine. Which is what you'll find in nearly all supermarket croissants. Even the ones advertising "all butter" in a supermarket will be using the shittiest butter that they're still legally allowed to call butter. Depends on the country but often has additives like food coloring and artificial butter flavor.
