I have seen this happening more and more lately - for bridal showers whoever is throwing it will put a theme on the invite of what type of gift to give them. Example of the most two recent ones, a garden themed gift ON TOP of the regular one and the one I am currently working on, we each were assigned a time of day, I got 8 am, so my gift needs to be something she would use at 8 am.
I can see why maybe they would think it's fun, but not when you're the person shopping who already had a gift in mind that doesn't fit your idea. Correct me if I'm wrong here, but even if you have a shower, that you obviously realize you're getting gifts at, shouldn't you be grateful for whatever gifts you get. I guess registries in a way do this, but you still have the option of picking through 100 ideas and you can always say screw the list and buy what you want anyways!