Wow, lukebuz, you have my sympathies. A freakin' trampoline?* What this sounds like is an attempt to show love (or curry favor) with the grandkids, and in the pursuit of that goal, the parents' wishes/feelings/needs are completely irrelevant.
On a smaller scale, the first Christmas after DW and were married, my lovely mother gifted us a Willow Tree nativity. Very nice, very expensive, very fashionable, ...and very much to my mother's taste. But not to ours. We were poor college students, living in a one-bedroom apartment, ...and already had a nativity. We didn't need an extraordinarily expensive (to us) nativity, didn't like it (creepy faceless figurines!), and had nowhere to put it. However, knowing that my mother took great care in her selection of gifts, I did not want to offend my mom. So I called my dad and explained our predicament--"I know it means a lot to Mom, and it's really nice, and I know it's really expensive, but it's not to our taste, and we don't want to offend her, etc. What do you suggest?", implicitly hoping that he'd give us good advice without telling my mom. Apparently, I was a bit too subtle, because he subsequently discussed it with Mom, and let us know we could return it to them. I did so, naively thinking (I was 22, cut me some slack!) that they'd find another gift, or just drop the whole thing, and no harm done. Nope. Mom was terribly insulted, although she never admitted it. Fortunately, a few years later, the burgeoning population of grandchildren reached the point where she was overwhelmed with the time and effort required to find just the right gift for each, and that's when she and Dad started sending money to their kids instead, with a note that basically said "We can't keep up any more, so please use this money to get something your family would enjoy." We have gratefully done just that ever since, and made sure to tell Grandma and Grandpa how their generous gift is bringing delight to our family.
*We had a 14-foot trampoline growing up (this was in the early 90's), and it was an instant hit in the neighborhood. I think two injuries resulted over the years, including a really nasty forearm fracture. Eventually, the insurance company convinced my parents to get rid of it.