I mean, everyone up-thread says "eyesore", but there are other concerns as well.
Chaining a bike to a random object can be a necessity; not everywhere has any bike racks, let alone ones anywhere near where you need them. Chaining a bike to a random object can be perfectly fine.
But people also chain bikes in ways that block access to sidewalks, buildings, and other bits and pieces of the public spaces. It may not be intentional; the way other people use spaces may be different from yours, and it may not be obvious that locking your bike to a particular place may make life more difficult for other people.
Chaining a bike to a random object can be a necessity; not everywhere has any bike racks, let alone ones anywhere near where you need them. Chaining a bike to a random object can be perfectly fine.
But people also chain bikes in ways that block access to sidewalks, buildings, and other bits and pieces of the public spaces. It may not be intentional; the way other people use spaces may be different from yours, and it may not be obvious that locking your bike to a particular place may make life more difficult for other people.