That's harsh but that's the law. It has been voted after some corruption scandal and is very strict. Politicians from a party supporting Macron have been previously sanctioned for exactly the same reason.
Le Pen's problem is that, as she is a autocratic party leader, she decides and give orders about the way the money is spent, which person to recruit, etc... so there is unambiguous written evidence. Unlike the current case about Sarkozy, there is no need for interpretation as there is significant written evidence.
The irony is that, like most other far-right politician, she's been complaining for years about corrupt and roten politicians and has been calling for a strict application of laws! π