The Durso Standpipe is designed and shipped in three parts.   The first part is the vertical pipe with the bulkhead adapter on one side.  The second part is the Tee/Elbow assembly and the third is the end-cap on top.

These are separated to allow you to insert or thread the vertical pipe into your bulkhead and then simply slide on the Tee/Elbow assembly.  If these were attached then you would need a much bigger overflow chamber to allow room for the standpipe to rotate while inserting.

Unassembled Durso Standpipes as Shipped

This also allows you to finely adjust the height of the standpipe if needed (glass covers for example).  You can order a standpipe a little tall and trim as needed.

Should you glue on the Tee/Assembly?  Absolutely not!  For the same reason, should you need to remove the standpipe for any reason you just slide off the Tee/Elbow assembly and remove the vertical pipe.