Perhaps I'm paranoid or overly sensitive about how people deal with their pets, but it makes me very nervous when I see an unsupervised parked bicycle with a dog sitting in it. The primary problem I have with it is that parked bikes are relatively unstable and I'm concerned that a careless pedestrian or cyclist, a gust of wind, or even a small earthquake will send the bike toppling and the animal will get hurt. None of these things are exactly rare in Tokyo. The other thing that troubles me is that most of the little dogs I see perched in these baskets are tethered on a short leash to the bike's handlebars. I'm sure this is done to keep them from jumping out, but I worry that this will just mean the dog will hang itself if it does try to jump out or at the least bring the bike down on itself if it attempts to jump out.
Seeing a pet sitting on a lone parked bicycle is not an uncommon sight and I think it's very irresponsible. I won't miss seeing this worrisome situation.