Why is the GULO pseudo gene expressed in humans?
I suggest thinking about this from a different direction — something like: "What selection pressure is there for loss of expression of the GULO pseudogene?"  

When thinking about that question note that the expression level is very low (less than 1 transcript in a million in the tissue with the highest level of expression.  

In general, evolution is messy and I think your null hypothesis should be that this as a vestigial trait.

In other words, unless you can show that there is a significant cost for the low level of expression seen from this pseudogene there is nothing to explain.