In many cases it seems to be, as there are a lot of non-domesticated species in circuses that aren't naturally dependent on humans. When training a particularly strong or aggressive species for a circus, a human may also resort to inhumane training techniques such as whipping. I've even heard of wild animals becoming depressed due to being deprived of their natural habitat.

There may be circuses that do not use wild animals (e.g. animals from a domesticated species that relies on a human carer) and may treat them well, though I can't say that I'm familiar with those.

Overall, I'd say that circuses are a problem for animals, if mostly due to the species they so often choose to perform with them.

It is disgusting, leave them in their NATURAL environment .

There are lots of humans in circus acts, too. You don't see THEM complaining!