Not according to the French law.

A crime is not something you evaluate morally. It has a very narrow definition that is codified by each society it is been judged in.

It's grounds for having your citizenship revoked unless it will render you stateless. People suspected of being on the way to Syria would get their passports confiscated.

Nine EU countries made similar rules, though I don't know how many still have it in effect. Any citizen/resident of Denmark is still prohibited from going to the conflict zones of Syria and Iraq.

Then why weren't they arrested for that instead?

I am not familiar with French law. Is that literally a crime or are you saying it as an expression?

Because it can be difficult to prove. There are/were no direct flights to Syria. So people would travel to Turkey and cross the land border to Syria.