It says that they believe they shouldn't be allowed to add users if their service level doesn't allow it.

What's not clear from the article is how the extra charges of adding users is communicated. If you have two users and you attempt to add a third, does the site say "Your plan allows X users. By adding another, your plan you will be upgraded to Y which is $99", or does it say nothing, allow you to perform the action, and hope that you don't notice the email some time in the future. If so then, I see that as a bit of a dark pattern, others may believe that's perfectly acceptable.