What to do when the professor does not stand up to cheating? I was the teaching assistant (TA) for a very introductory physics course at a large state university. In the course of grading for this class, I came to learn that my university used the same course material as many other universities had for the equivalent course for more than twenty years. The answers to questions, in particular, were readily available online, through various forums. The syllabus explicitly forbade providing disingenuous answers, as well as any plagiarized responses. To be clear, this instructor was a fully tenured professor; not someone in any risk of losing their position.

After noticing patterns in homework assignments, I began to look for them in midterm and final exams as well. What I initially thought was a couple of bad students turned out to be more than 50% of the class regularly citing online sources word-for-word. Once I realized what was happening, I informed the professor, and I started assigning zeros to the offending students, in accordance with University policy, and the professor did not object to my assignment of zeros. However, all of this was still subject to the lead professor's review.

The professor asked for documentation of the offenses, and was in fact the undergraduate director of the department. I spent the better part of a week accumulating evidence, scouring Yahoo answers and other common homework repositories for the sources of the dirty students' answers. I found a veritable source for every single one. I printed copies of the students' responses, alongside their internet sources, and deposited the six-inch-tall stack of documents at the professor's door. 

As far as I know, nothing happened to a single student, and word was never made public of the massive cheating scandal that was blatantly obvious in this course. Multiple athletes were in this (1A) course. 

Is this just academia?
Beer and Circus calls this the "student-faculty non-aggression pact":


Faculty provide an easy class and don't look too hard into cheating
Students happily take the easy grade and leave the professor free to do research


I wouldn't say this is "the rule"; plenty of faculty do an awesome job teaching. But, I'm not surprised to hear your report -- some faculty don't care about teaching generally, while others care about teaching but don't care about grades or catching cheaters. Many feel (for better or worse) that cheaters get what they deserve eventually, and don't want to be personally involved in punishing them. 


  What to do...


I would carefully check the department and university policies -- often, cheating cases are explicitly prosecuted "at the professor's discretion." I would also speak directly with the professor. Speaking of....


  I started assigning zeros to the offending students...[collected evidence]...and deposited the six-inch-tall stack of documents at the professor's door.


I suspect this was not the best way to go about it. I would recommend first speaking with the professor, making them aware of the problem, and asking how to proceed. At a minimum, this could have saved you a week's worth of wasted effort. As it is, you assigned a bunch of zeros, and it sounds like your professor was okay with this -- that is some punishment, at least. 


  Is this just [the way of] academia?


Well, it's certainly a bit dysfunctional. And it's inevitable as long as prosecuting cheaters is a lot of work for the faculty while providing absolutely no "reward" when done successfully.