Why are the tapes marked overwriteable?

When I put in expired tapes to the tape machine, Backup Exec is marking all the tapes overwriteable and linking them together. I only have 6 tapes availible to write on to. I used to have only two overwriteable tapes. Why is Backup Exec making all the tapes overwriteable and how do i reset the tape to normal so it can be written on. 

We are running backup exec 2012 on windows server 2008 r2 standard.