It isn't a simple thing to ask. Homeless people have to carry all their possessions when they leave their camps, to prevent them from being stolen by other desperate people. So they're exhausted. 

It is also unreasonable to ask homeless advocates to do what you suggest. We don't have the time, energy and resources to do that. It's a community responsibility. The city should be providing trash bags, pickup and portapotties or simple cheap camp toilets that put the waste into plastic bags to be disposed of. 

Better yet, the city should provide more rest stop camps, but NIMBYs in the neighborhoods keep blocking potential sites and the city doesn't have the guts to override them. 

So the NIMBYs are getting what they deserve. You can either have legal well run camps or illegal messy ones. Take your pick.