I'm a bit of a computer history buff, and dig around through old software archives from time to time. It seems to me that developers back in the 80's (at least in the PC / home computer world) were quite possessive of their source code, even for hobbyist projects that they gave away for free. It might seem a little odd to us today, although perhaps they had some rational motivation (e.g. hopes of productizing it or selling the IP later). So I'm not surprised that this program is also closed source, with some vague integrity/security reasons offered.

I say this with all due respect to the developer, who of course is free to distribute his software however he would like. It's a very cool project, and I often wish we could see a resurgence of such simplicity and distraction-free UX. :)