People minify js files because they can.
When you already have a pipeline like gulp or grub (and if you are developing JavaScript, you should), adding minification is just one more line of code. So, why not minify if it doesn't represent any effort and it could mean better performance? Also, usually, developers don't have access to the production webserver.
Most of the time, people are to busy to analyze all the pros and cons of every action they do.
Said that, if Debian is an open source OS, then yes, they should have access to the unminified code of the applications they include. Or, at least, the sourcemaps.