This is not a software, therefore it cannot be proprietary (they just
selling/licensing them, which is perfectly ok). Proprietary means when the
author release a program, without "providing the source code under liberal
terms" (i.e. those respecting the user's essential rights - the four freedom
rules). If the application became popular, there will be many users grow
into, and depending on it (this is the main problem why the transition from
proprietary software is hard most of the time).
Now, the problem is, that the author became some sort of "God"; he/she can
see the code/modify the application however he/she wish, even put backdoors
into it, since the compilation process mask every trace of his/her mischief.
The only neccessary task here is to ensure, that the application stay
popular, so implementing new features (even if they doesn't benefit the
users/make the application better), using new technologies, and making the
overall look fresh and attractive is needed.
That's why you can see complex proprietary applications, provided
free-of-charge (aka freeware), so that the companies make you/other people
accusomed into their particular product. Companies needs to have control over
the market, and people who use their "products", because information means
power (and money).
Hope this help you better understand the issue.
