What is to prevent someone from dumpster diving to collect dna?

While that dna is anonymous if anything interesting turns up in the dna it would be possible to trace it back to the owner with some certainty because if it was, say, at a restaurant you could (assuming the use of credit cards, security cameras) be tied to patrons of that restaurant or workers. And at that point you could triangulate enough to have a reason for further investigation and/or get warrants for things. Perhaps (ianal).

Taking this further how would something like that be viewed? I'm curious to what extent that would be viewed as unethical (in the US SCOTUS already has said you can pick through trash). So it appears to be legal.

http://en.wikipedia.org/wiki/Expectation_of_privacy