True, but that would just change their tactics. The trouble with "tax-the-spam" ideas is that this is all computer-based. Spam itself is computer-based. That makes any effort at controlling it via gov't very iffy at best, and dangerous at worst. Tactics always change, and spam will always go over the wires.
The best way (IMHO) to eliminate spam is to educate the end user and prevent any clicks or consumerism of the products/services being hawked. Make it a waste of time and effort to send spam, and it goes away.
Im against the tax, just though I would point out there are ways tim impliment it.