"I'll second this. As a guest, my only job should be to RSVP. I shouldn't have to go out of my way to ask where the MTB is registered or search for the information."
You're correct that your only "job" is to RSVP. However, buying from the registry is a preference and not an obligation, and not everyone is interested in the registry. It is generally considered rude to push gift preferences on guests, particularly those who haven't even said if they're going to attend, If anyone is interested in knowing, they'll ask.
On those few occasions that I've received a shower invitation that includes registry information, I just take it as an indicator that the host does not know the finer points of etiquette and that's that. It's pushy, but it certainly doesn't reach the level of "pay for your own lunch at the shower" or not sending a TY note.