I've seen this before on HN. It's frustrating that it still happens. Obviously, if they can delay paying you to 60 or 90 days (but still get paid themselves at 30 days) they make money, but it's sleazy. Perhaps people should start naming and shaming companies who do that?
Luckily, the EU has law about this.
For small businesses, putting off payments to suppliers can be the difference between making payroll and going bankrupt. You can hardly blame them.
I make a point to pay as soon as possible, on the theory that this will improve the service I'm getting from other professionals ("Let's do Jacques's file first, he always pays promptly and we need the cash").
Also, the 60 to 90 day payments are typical for companies that like to keep things "lean" ... they want to generate revenue before they pay. Dell was known for this with their hardware purchasing - they would maintain a negative cash conversion cycle because they could sell a computer before its components were even paid for.