perhaps the ideas that you happen to be giving them have already been considered.
Really? All ideas from multiple different people? Well, if that person is such a genius that he considered everything, then it would be very appropriate to list what he already though off and didn't like, so people can think on something else or not waste time.
Think about that. Really think. You want someone who has thought something through, then ended up at an impasse, to list each and every thought that they've had leading up to the impasse?
The list would be too long.
Personally, I try to consider what is suggested to me, and see if I can't twist it into an answer I can use. And I try to be nice from either end, unless the other guy was an ass first. But that's me.
On a base line, and being out of context though: I agree with you. It is annoying if they put down many responses with no further explanation or clarification on what they want.