Color my mind blown.
The other week I reported on a debacle involving a guy who put some allegedly non-recyclable goods in another guy's recycling bin. There was a lot of debate about whether it's legal to put stuff in other people's bins.
The answer, according to city spokeswoman Heather Burke: "It is not illegal to put recyclable material in someone else’s recycling cart if it is in the public right of way, as the carts are Denver’s property."
In other words, you can't go into your neighbor's yard to dump stuff, but if it's on the street, sidewalk or alley, go right ahead. This applies to trash cans too, Burke confirmed.
"In fact," Burke added in her email, "Denver Public Works encourages residents with extra recyclables to work together with their neighbors to share their recycling cart, if there’s room."
However, if you're worried that people are messing up your recycling with non-recyclable stuff, you can call 311 and the city will try to deal with it. Also, if you store your bins on your property, as the city advises, people may mess with them less.