They usually just reject the invoice with late fees and quote their stated 90 day policy. Also usually they will redline it if you try to put net-30 and just change it to net-90 anyway.
It's up to you if you accept 90 days in a contract or not. And if you accept 90, then it's absolutely within their right to wait until day 90. Why would they not? It was agreed up front.
Sure, and I had to accept it because they would bully me into it. "Here is a contract for $200,000 and you'll bill us $30,000 each month, but we'll pay you 90 days later".
They know I don't have a choice to turn down that work just because I don't like the 90 day rule, and they only have that rule so they can keep money in their bank account longer.
And when I was on the corporate side, the vendor specifically would not do the work for me because they didn't agree to the 90 day terms, so I had to fight with my own people to get them paid faster so I could work with them.