I think the reason it is illegal is purely a public liability insurance issue.

I've seen lots of stuff disappear from piles overnight in our area. I've never worked out how to find out when they are on so we've never put anything out. Our neighbours were moving and put a few things out with a sign saying "free". They mentioned it to us thinking it might be suitable for the kids but when we looked it wasn't something we could use. We went out half an hour later & it was gone! I've never taken anything personally only because we have too much stuff already.