A certain number of businesses will just pay an invoice if it looks real and has a business justification. There are scams around this, but also vendors who may be taking advantage of this. If 30% of customers will just start paying the invoices, it's worth it to them to deal with the questions from the rest of them. At least until reputational damage starts.

A very simple example of a scam that's probably happened to you if you own a domain name: as the expiration date approaches you'll get "invoices" from companies for domain name renewal charges. If you read the fine print it will say "this is a solicition for business" but otherwise it looks just like an invoice. Some people will just pay them.