Good question. This is something that has been recently changed so I think some aren't sure what the new rule is. They didn't have a limit before, but a lot of hubs were getting filled with Amazon products and starting to look like spam. With the new Google Algorithm set up, they changed it. Now it is 1 amazon product for every 50 words.
However, if older hubs have to many amazon products from before the new rule, they should have a warning for the hubber to see. It might be either the hubber isn't aware of the new rule, or it was done before the change. In that case, they might not have fixed the error yet!