Yes, until that is your money you owed you're fine with stealing. You are STEALING from me by not paying me for my services and work already performed.
Don't you get it? I MUST PENALIZE stealing of my services otherwise everybody would think it's OK to steal my labor. Your late payment is costing me money and I will get that money from you.
There are consumer protection laws out there. I don't know about the US, but in Australia, excessive late fees can be brought to court via the ACCC.
If you have ever studied any contract law, you will know that unreasonable terms can get voided.
Unfair Terms
Unfair terms legisalation is relatively recent in Australia; to date, only Victoria has comprehensive unfair terms legislation in its Fair Trading Act 1999. That Act renders unfair terms in consumer contracts void (s 32Y). A term is unfair if 'contrary to the requirements of good faith and in all the circumstances, it causes a significant imbalance in the parties' rights and obligations arising under the contract to the detriment of the consumer' (s 32W).