Surely ttf -> png is like dvd->vhs, the font (and any other artistic work) doesn't magically become your property due to a format shift.
You'd be surprised.. Like Grey Alien mentioned, doing TTF -> PNG shifts it from a 'computer program' to a 'typeface', which have different legal protections. It doesn't automatically always make it free everwhere, but they are definitely considered entirely different cases.
TTF is considered to be a 'program', since it contains instructions to 'run' for the computer that generate a specific output (the letters in question), which has different protection than just a bitmap containing the output of the 'program'.
It's closer to spreading a screenshot of another program vs. spreading the actual program itself, plus there are seperate rules dealing with typefaces, which pretty much predate computers altogether. showing a typeface in a computer program isn't much different from using a typeface to print a newspaper or brochure, just to name something. A newspaper can print in pretty much any font they choose to as well.
Similarly, MIDI files supposedly are considered to be sheet music (which is a set of instructions), and not a sound recording, and because of that have different protections associated with them than a .wav or .mp3. Not sure if it still applies under the current copyright laws, but there used to be cases where it was legal to use the one royalty free but not the other.