Why can't they charge for it if it's currently free? Technology companies have a long history of starting out as zero-cost and then going to a fee-based structure. Why else do you think the $9.95/mn dial-up ISP is called NetZERO? Similarly, they may limit their mini-messages to 140 characters, but that does not mean they can't or won't inject ads into the messages. And sending standalone messages would not be considered spam because the people receiving the messages have a business relationship with the company (no, I personally don't believe that means "it can't be spam", but the US system of "justice" believes it).
So how can they make money? Here are a few ways:
1. Charge people to create an account (I'm assuming you need an account in order to create a message).
2. Charge a per-message fee (even if it's a small fee such as one cent).
3. Charge per-message fees to allow users to send larger messages (I'm assuming you mentioned a 140-character message because that's the size limit they impose on the message).
4. Sell advertising space on the page where users create the messages.
5. Sell advertising space to be delivered with the messages.
6. Sell access to the system so that advertisers can read the messages and develop targeted advertisements (either targeted to the author based on all messages they've sent, or targeted to the recipient based on all messages they've received).
7. Charge for access to the API.
These are just the ideas I came up with in about 10 seconds. I'm sure if I actually cared enough to think about it, I could come up with a lot more. But suffice it to say, there are plenty of ways to make money. The question is -- would people continue using the service if they had to pay for it?