Why is spam bad?

There are several reasons:

1. The free ride. E-mail spam is unique in that the receiver pays so much more for it than the sender does. For example, AOL has said that they're receiving 1.8 million spams from Cyber Promotions per day. Assuming that it takes the typical AOL user only 10 seconds to identify and discard a message, that's still $15,000 of connect time per day spent discarding their spam, just on AOL. By contrast, the spammer probably has a T1 line that costs him about $100/day. No other kind of advertising costs the advertiser so little, and the recipient so much. The closest analogy I can think of would be auto-dialing junk phone calls to cellular users, and you can imagine how favorably that might be received.
2. It's all garbage. The spam messages I've seen have almost without exception advertised stuff that's worthless, deceptive, and partly or entirely fraudulent. (I include the many MLMs in here, even though the MLM-ers rarely understand why there's no such thing as a good MLM.) It's spam software, funky miracle cures, vaguely described get rich quick schemes, dial-a-porn, and so on downhill from there. It's all stuff that's too cruddy to be worth advertising in any medium where they'd actually have to pay the cost of the ads. Also, since the cost of spamming is so low, there's no point in targeting your ads, when for the same low price you can send the ads to everyone, increasing the noise level the rest of us have to deal with.
3. They're all crooks. Spam software invariably comes with a list of names falsely claimed to be of people who've said they want to receive ads, but actually culled at random from usenet or mailing lists. Spam software often promises to run on a provider's system in a way designed to hard for the provider to detect so they can't tell what the spammer is doing. Spams invariably say they'll remove names on request, but I have yet to hear of any that actually do. Indeed, someone I know registered a newly created address on a "no spam" web page, and promptly started to receive spam at that address. Spammers know that people don't want to hear from them, and generally put fake return addresses on their messages so that they don't have to bear the cost of receiving responses from people to whom they've send messages. Whenever possible, they use "disposable" trial ISP accounts so the ISP bears the cost of cleaning up after them. I could go on, but you get the idea. It's hard to think of another line of business where the general ethical level is so low.

Spammers do more than spam