I don't know about France but I'm sure in Italy there's a limit[1] similar to that (no more than 1000-1200€ per month) because on internship ("stage") the company is not paying a lot of taxes - almost nothing - so you can't expect to allow them to pay an intern 3000€ per month as the tax saving would be huge and they'll use crappy contract more than they're already used to do.

[1] I'm not sure it's a hard-coded limit in the labour law or if it is something more related to the fact that if the wage is "excessive" you could have problems with the tax authority.