It is true that many laws can't effectively reach people who are operating completely outside the country, there are two things to remember:
First, because most spam advertises goods or services offered by North American-based entities (for example, get-rich-quick schemes and quack medical remedies being sold out of someones basement), we advocate anti-spam laws in which the focus is not where the email came from but on whose behalf the spam was sent. If the law applies to the advertiser — the entity profiting from the activity — it doesn't matter where the spam originates.
Second, the reach of a law outside the borders of the country it is legislated in is tenuous at best, however that fact does not negate the need for or effectiveness of laws against those breaking them localy. It can be very difficult to bring a murderer to justice if they escape abroad, but no one could seriously argue that this fact means domestic murder laws are unnecessary or irrelevant. Spam isn't comparable to murder, but if our judicial systems means anything, the same principles of justice should apply.