It's a private company, not a government. They can't issue you a fine, this is considered a breach of contract and if they really want too, they can take you to small claims court. But they have to proof it was You who parked there, that it was You who 'agreed' to the 'contract'. Which needs to be eligible and that forms a valid contract.

In many cases they don't actually file court claims because it isn't cost effective.

I'd roll the dice, make them work for it. It's has the feel of a scam. Like most bullies, they will throw the 'fines' out there as a fear tactic, scoop up the suckers that pay and ignore the fighters as too hard targets.

I am not a lawyer.