Neither does charging. Most SAAS products charge on a monthly basis, so the most any vendor owes you is 30 days of usage. Even so, they could always just shut down today and refund you your money.
Not to toot our own horn, but Fog Creek has been around for 12 years, we've been profitable every year along with having dozens of employees. There's little chance that we're about to disappear (or get acquired). We've invested a lot into Trello, and we're not going to change our minds on a whim.
We realize people are running their businesses and adapting their workflows around Trello, and we're encouraging that. Our plan is to give you even more security and more reasons to integrate our product into your business in the future.
So while I can see some merit in arguing that "saying always be free" is just a promise, it's backed by a company that's over a decade old. I don't see any evidence to suggest the scenarios you are describing.