I sent an email to the group here at UW that manages mailman. This is the response 
I got:

The RFCs don't actually mandate the usage, value or actions for precedence ("the 
use of Precedence is not specifically recommended by this specification", or 
"Precedence: Non-standard, controversial, discouraged"), so we stick to what our 
experience tells us. Most emails with this tag are things such as "Out of office" 
or vacation messages. Accordingly we drop emails that have this tag to save our 
users from unwanted emails.