I don't particularly have a problem with giving tasks where the poster already has a solution. I have done so myself. However, I agree with others that if you had several days to golf your own solution then you should wait a while before posting it.
At least for me golfing often involves first solving the task, then finding bits and pieces that may be optimized and I then gradually move towards a golfed solution. I had moments where I thought I'd be done and nothing else would be possible anymore and then, all of a sudden, I shave off another 30 characters.
Giving others such a golfed-to-death solution beforehand might discourage them from even trying. Also, since the own solution is often essentially finished it is not really part of the usual competition, where one would notice that another solution is 5 bytes shorter than the own and then see that as motivation to beat it.
In case of my own question above I gave my own solution I prepared months ago only after someone beat it. In case the poster has the shortest solution even after some time has passed, I guess it should be posted, but not immediately after posting the task.