Since it was just the grandparents who asked, but the reply was sent to "everyone," I think that falls more on the rude side. You, personally, didn't ask; maybe you had your own ideas about what to get the kids, or weren't planning to get them anything, or whatever. I think it's established that it's rude to send gift ideas without being asked first, right? Of course, in general, there's always the possibility of a miscommunication or technological snafu--not sure if that's applicable here.
Beyond that, for me it also depends on how it was worded. I like DavidH's comments--that if they'd mentioned the kids were saving up for X from StoreA, so gift cards to StoreA would be appreciated--it would be more palatable. The wording as described by the OP (plus the fact that she didn't ask for the list in the first place) makes the parents seem rather presumptuous.
I think I would just get the kids whatever I'd been planning to get them before. Or, if I didn't have any ideas before, maybe something related to the list items, like a hockey-related book for the hockey player. Something I thought they'd like, but not something I felt was dictated to me.
My friend was just telling me how her 13-year-old niece's wish list had nothing on it less than $100. My friend had budgeted $25, so she just sent that as a gift card. But it left a bad taste in her mouth. Not that someone should put cheap junk on their list that they don't want, of course, but I think there's gracious ways to get the message out to people, and ungracious ways. Like if you know you only want expensive stuff, go the "saving up" route, which implies you're going to buy it anyway regardless, not that you're expecting someone to provide it for you.