E-mail harvesting, which is what the action of collecting "good" email addresses is called, is sort of common, but not likely.
Most likely, and most commonly, people fill out their email address in certain forms on certain webpages, and forget to uncheck the box titled something to the effect of, "Check here to tell us that you DON'T want to receive unsolicited email from us," and then their email went on that company's list, and that list got sold to someone else... and someone else... until, eventually, you're getting email from companies you've never heard of, never contacted and would have no business contacting.
Just one... little... checkbox... can set off this chain reaction and succumb your email address to a life of spamming.