Yes in many cases BIS may recognize that a new PIN is associated with your SIM, but that's not always the case. But this is driven by the BIS account/PIN. The PIN on the BIS account is changed when you set up the new device. This happens with you log in and transfer the email account to the new device. So yes you are correct, once you set your emails up on the new BIS account it automatically unlinks them from the old one, thus not having an issue.
The issue is when a person never sets up another blackberry linking their email accounts to the new device. Everyone that gets rid of a blackberry doesn't get a new one. So if they haven't done this, in many cases their emails goes to that device because it's still linked. So they never unlink it which means the person who now has the blackberry cannot add their accounts because the BIS/PIN on that device is associated with someone else. A SIM doesn't transfer PIN to another owner because then anyone can steal a blackberry put their SIM in it and then use it. SIM just registers the device to the network but the emails are changed by the method you stated above. Everyone does not do that though, hence the issue. It is fixed by either having the original owner manually delete the PIN from their BIS or calling the carrier and having them reset the PIN so it's not attached to anyone's BIS anymore. This is not just something a few people have experienced. It's been something specific blackberry for a long time.
Not sure how many links I have to post for you (including from this site) to see that it's been something well known for a long time.
Email account problem
So at the end of the day, if the previous owner does not remove the blackberry from their BIS or does not change devices, this is a recurring problem. Of course it's just a phone call to fix, but it is an issue. Again, it's not a big deal if people know what they are doing but everyone doesn't. I remember buying a 8310, 8330, 9000 and 9700 from people and for all of them I had to call AT&T and Verizon to have the PIN cleared from the previous owners BIS because I couldn't set up my email and I was getting all of their email.