"can't. free press. if we censor this, then other things will get censored, and we'd lose the freedom of speech and press."
Actually, unsolicited commercial email *is* illegal (see, e.g., the CAN-SPAM act), and ISPs *will* shut down accounts that are sending it. Several high-profile spammers have been brought up on criminal charges.
The problem is that most of it is sent by computers that have been compromised with trojans or viruses. By the time an ISP notices that one of its accounts is acting funny, it's already sent out a couple million spam emails. They can shut it down, but the spammers will just buy more systems from a botnet and keep going. Not to mention that some ISPs in other countries probably just don't care. You can't just start not accepting any emails from, say, every ISP in Eastern Europe.