Sublime Text isn't free; it's sold on the honor system:

"Sublime Text may be downloaded and evaluated for free, however a license must be purchased for continued use."

The developer could try to enforce this rule by building in DRM or by threatening legal action, but those things would make for a pretty crappy user experience. Instead, the author chose to do the right thing by users and provide an evaluation version with no restrictions.

Legally and technically, you can certainly use Sublime Text without paying for it. To me, though, using Sublime Text for your everyday work without paying for it is the ethical equivalent of using a cracked version of Photoshop.