Its quite interesting the mention of spam, but what's the difference emailing and dropping in a leaflet?
Because if everybody sent spam, the internet would grind to a halt as servers became overloaded, companies would have to pay employees to filter it all out/delete the posts, a company with a relatively small capacity server could become so inundated that it could interfere with their ability to trade profitably.
You see, although you might only send 30 or 40 targeted emails per day, it's not just about the ones
you send. It's also about the millions of spams for viagra/fake rolex watches/date a ditzy blonde/buy shares in Nigerian oil wells/send £50 to claim your lottery preze etc. etc. No distinction is made because there could be a big fuzzy area between what is legal and what isn't. Therefore, the simplest way is to make receiving emails an opt-in exercise rather than opt-out. If you think it through, it's the only practical way to do it really.
Furthermore, if you start sending unsolicited commercial emails, if enough recipients start filtering your email address to go into "blocked" or "spam" folders (on webmail accounts rather than hard drive filtering), eventually the domain name (i.e. your service provider's name) can go onto a banned list. No legitimate ISP would want that to happen of course and they would have every right to terminate your account. In reality that would take a lot of spam to get on such a list. What would be more likely to happen is that a recipient could complain to your ISP and if it was relatively minor, they would give you a final warning.
The other issue is that bandwidth costs money. The argument could be "Why should the ISP have to increase their prices to everyone so that some of its customers can send unsolicited commercial email?
However, dropping a leaflet in is less intrusive. OK so someone will be paid to pass it up the food chain or drop it into the recycler but that is as far as it goes regarding effects on others. No-one else is subsidising you for it (like paying for extra bandwidth).
Not sure how much bandwidth I've used posting this either