Well, I take it that /he's/ promising not to spam. The same that I can promise that no crimes will happen in my home- it's only as good as long as it's _my_ home, not someone else's, and I can reasonably manage things.
Even with a friendly-reading T&C he'd be free to break it any time, as would any future purchaser. I'm not a laywer but I'd believe T&Cs exist more to protect a site than its users, presenting the general terms and conditions the users can expect a site to be presented under, not a guarantee of of the same.
Pre-existing contracts may be binding on someone who purchases a business. So the buyer of a web site might be bound by the TOS. Thus, a promise to customers could survive the company changing hands. (I can't speak to the particulars of this case, though.)
The flip side is that TOSs typically have some provision for amendment. Such provisions might be invoked by a company's new management. However, the management has to be careful even then, because of this: http://en.wikipedia.org/wiki/Standard_form_contract
Again, I'm not suggesting that any of this would necessarily apply to threewords.me. This is just my understanding of how it works in general.
BTW, I couldn't find a link to the TOS for threewords.me.
Even with a friendly-reading T&C he'd be free to break it any time, as would any future purchaser. I'm not a laywer but I'd believe T&Cs exist more to protect a site than its users, presenting the general terms and conditions the users can expect a site to be presented under, not a guarantee of of the same.