Mumma - it is a policy but every hospital i have worked in!.
Basically it is a time and sterility issue.

Once you have been given your spinal the ob is usually scrubbed up (the midwife isnt as they are not part of the theater team as such). And your body and the table is sterile. SO to have someone come in to catheterise could cause a sterility problem as wee runs out etc etc (although it is considered sterlie as well). As you will have no control down there - there are a lot of precautions to be taken so that your legs are positioned correctly as not to fall off the table.. it more people touching you - reducing sterlity. But also the anaestist would prefer you to have your spinal for the least amount of time possible - and not be waitin around for the catheter to be placed in case they make a real mess and then they have to clean everything and re steriliser the theatre area .. which is very difficult with a patient who cannot move from the chest down.

A lot of hospitals now - catheterisation doesnt need to be sterile. ANd honestly yourmidwife would have done a million of them and should only take 5 minutes after your prep shower. then the theatre staff can just get on with it.

In saying that though.
They are often put in theatre if the ob agrees the nursing staff to do so. (not just c/s but all operations that reduce mobility in the patient post-op)
So it is definatly worth asking about it - it is your body