As I read the article, the homeless were squatting in a ravine situated on private property. Of course the owners of the property would be responsible for the cost of the clean up. The larger question is why the HOA and management company didn't know or police their property boundaries, evicting the squatters before it turned into such a big problem.
I agree with the homeowners that the management company should share in the cost of the cleanup.
Homelessness is a huge, growing headache in the US south, west coast and urban areas. It's disastrous for residents, businesses and most of all, the unhoused themselves. How and when this is going to be turned around is impossible for me to imagine. Can't blame this on either political party either. It's gotten steadily worse since the 1980s.