Yup, I figure they're put out there for the city to collect. If I get them sooner, it saves the city some labor and they're less likely to kill the grass or get snowed on before they're picked up. Unless the city composts them and sells them for a
profit. Then I might be stealing.... Maybe checking with the city would give you an idea? If you really want to ask permission, just do your scavenging on a sunny Saturday around noon. Drive around and ask permission of the people who are actively raking and then come back to collect the bags as you can.
I know a guy who has a chipper/shredder just for processing leaf bags. I just put them whole into my
chicken winter
greenhouse and they get broken down by the birds. For my garden I use my grass catcher on the riding mower to chip and collect the leaves off my yard. If I needed more leaves for the garden, I'd probably dump the leaf bags in a long line and mow them up.
We just got jumping worms in our area so I'm trying to not take leaf bags any more. Which really sucks. But when I did collect them, I'd only grab them from lawns that had weeds in them which I figured meant they were less likely to have gick in them.