I don't understand how academic authors get away with this.
Papers that I implemented or tried to implement in the past were often sorely lacking in details. Your point about unstated assumptions is absolutely true. Some papers are nothing more than glorified abstracts.
Free access to PUBLICLY funded research should be the default.
I think the issue is that the current incentive system make it generally much more favorable to not publish actual source.
-A trivial bug in your program can go a long way to discrediting you if someone wants to.
-If your methods get inlined into a popular library, the number of people who cite your work will drop to 0. Popular libraries (for Machine Learning at least) typically have a set of authors who will be cited, and if you implement some new method or improvement in that library, they will get the credit and you won't. Since being cited is the most common measure of your net worth, having your work be accessible only with your name attached is significantly better for your career than having your work be maximally accessible.
I've had more than a few situations where the author does release the code...and it doesn't match the paper. The equations are different, restrictions are tighter, etc. It's the single most frustrating aspect of my job (excepting autotools).
Releasing the code is another way that people can attack your conclusions, so maybe people are reluctant to do it?
There's not enough room in a 12-page paper to give details. As someone else mentioned, every detail is also an invitation for some nitpicker to try to sabotage your work.
When I did my diploma thesis my mentoring professor explicitly told me to only include short excerpts of the code, if any. Okay, it was Physics, but still. Having read the thesis 1 year after I handed it in, I realized the code fragments are mostly usely/trivial for the reader. Either you publish the whole code or none of it.
Papers that I implemented or tried to implement in the past were often sorely lacking in details. Your point about unstated assumptions is absolutely true. Some papers are nothing more than glorified abstracts.
Free access to PUBLICLY funded research should be the default.