That's not greedy, not really. All it means is that they found something at one store that they couldn't find at the others.
I can see me doing it. I register for china at the store that has the one pattern I like; it also has an online presence--great for out-of-towners. I register for towels and sheets at someplace basic like JCPenney or Sears (or, maybe there are towels at Pottery Barn that are EXACTLY the color I want). But the cookware I like is at its best price somewhere else, or they have the best arrangement of pans in a set. And I register for home-decor stuff at Pier 1, bcs that's my vibe.
I might also choose more than one store bcs people in NYC can't get to JCPenney easily--there is one, but it's a pain to get to for many people; they can get to Macy's. My suburban family would find JCP easier.
Helps you, really, in a way:
"If you need gift ideas, see their registries at X, Y, and Z. There's a wide range of suggestions to choose from."
You can add, "Call me if you need further ideas that aren't on their registries." But then everybody will bombard you, so maybe not.
but I like the "if you need" bcs it implies that you aren't expected to use the registry; I like "gift ideas," bcs that's all the registry is, it's not a shopping list or a direct request for you personally to buy any or all of that stuff.