We have a pretty good national do-not-call list, but spammers are trying to challenge it as a free speech issue.
I expect them to attempt to confuse the antispam legal efforts the same way, so …
Your inbox is an entry to your personal space, and there's a social contract regarding how other people use your inbox. You expect anyone placing email there that they have some relationship with your or other expectation that you'll find their email of value. If that's not the case, it's trespass.
In addition, if they do so, and you read that email, they've taken a few moments of your attention and time, which has actual financial value. That means that spam is also theft.
I'm pretty sure that's not a conventional legal interpretation… but how about some common sense?