I work in a ticket office - there's next to no effort involved in checking the restriction code on any ticket that has one.
However, there are a lot of retail and revenue staff who don't understand the concept of a restriction code. As such, they won't be able to perform this task.
I was minding my own business one day, walked past a ticket office (for a TOC I don't work for) and was pursued onto the street by the clerk - he was unhappy because he was unable to book an Advance ticket for a passenger (apparently it's normal to tell passengers they have to do this at a nearby large station) and asked if I could show him how to do it. During the process, he said something about all Off Peak tickets being valid only after 0930. I then taught him about restriction codes and how to look them up. He said that in several years of being in the job, nobody had ever explained these things to him and that he couldn't believe how simple it was to find out this information. He shook my hand and I continued on to enjoy my Saturday afternoon. I gave up about 20 minutes of my time to help out and I'd gladly do it again.
He isn't the only one in this situation. I have great respect for the fact that he had the motivation to ask. Few people in customer service roles actually want to be unhelpful to their customers, such as in the manner described in the OP. Staff's better nature is often failed by training programmes which don't do the system they're working with justice.
Oh and I used to work in the information point at Euston and would give out printouts from the retail manual for anyone after restriction information of that kind. The standard practice however was to send people to the ticket office for information on prices and less common restrictions.
Considering you don't actually know who the op asked, don't you think you are being a bit harsh? Doesn't Euston have a Network Rail Information point?
It's normally staffed by Network Rail security, a Virgin Trains floorwalker and a London Midland floorwalker or ticket office clerk. It is a collaborative initiative with aim of enhancing the provision of information by having "one of each" to help each other out. That said, politics meant that the logins provided for LM and VT had no internet access, significantly reducing the usefulness of the facility. It was still like that when I left, though I gather it has been sorted now.