There have been cases in the UK, I believe, where a couple picked up a discarded winning lottery ticket and tried to claim it. I think they got a suspended sentence, at least.

IIRC, video surveillance showed that it wasn't them who had purchased the ticket. (Or at least that they couldn't have.)

I don't necessarily agree with it, but that is the law in some countries/jurisdictions.

In the US, if the lottery ticket is signed it then belongs to the signer

That has nothing to do with purchasing it.