"Oh, we can recreate this, only better or more of them"
"Oh, this missile uses IR, we can come up with a counter"
"Oh, this device uses such and such chip, our hackers know an exploit for that one"
Come on, use some imagination.
I'm thinking back to the Cold War - one way of seeing the rivalry between the US and USSR is as a game of making the other side spend more and more money, in hopes the enemy's economy will give out and collapse before yours does.
The only real downside I can see of public manuals are the risk for adhoc use of captured equipement. However I believe the upside of having the operators being able to use the equipment properly easier is greater.
You can't actually view it on the Internet, because as it notes, it has a dissemination restriction to US government agencies and contractors only, but it is unclassified and the soldiers on the crew are employees of a US government agency and entitled to read these. Reading paper is fine. In fact, when you're actually on a tank, it's your only option. You're not usually going to have a ruggedized computer that can access the Internet with you, if there's even a network at all serving wherever you happen to be. Information needs to be available in the absence of connectivity.
So these leaks in all likelyhood are not militarially damaging in anyway, but may weaken public perspective of the military (incompetent with keeping 'vital' information safe for instance).
I see no particular reason though to openly share these documents from first sources either. Security through obscurity is bad security on its own, but can be an extra layer when combined with other tactics.
"Oh, the armor is weak in the back?"
I have seen the same problem in companies, where manuals and reports are hardly accessible and thus effectively lost in the name of confidentiality ...