Spammers who use faked header information are engaging in fraud and should be charged with such.
When a spam that pitches some product or service purports to be from xyz.com but actually comes from abc.com then the sender is guilty of fraud. In effect they are using a fraudulent document (an email) to obtain pecuniary gain and I believe that's already a crime with significant penalties available.
The problem is that authorities just aren't interested in pursuing such fraud. Why then, should we think that they'd be any more interested in prosecuting spammers -- even if we do have spam-specific laws?