1) If the mail you receive actually interests you, you wouldn't classify it as "spamming" ("Did I really sign up for that? Anyway it sounds good"). If not you may avoid them in the future, but you would not have bought their product anyhow.
2) Scale: If a (real) spammer sends out 20 million emails and 0,0001% buy viagra. Thats fine for him. The other Emails cost him close to zero, so every sale it profit.