I'm sympathetic to this concern. Seeing "code of conduct" plastered around a site doesn't give me any great confidence that it's a good place to be - it's like seeing "no formaldehyde" on a bottle of milk, it raises more questions than it answers...
...Of course, at one time the answers to those questions would've been common knowledge, and such a notice would've been of no small value. There's a reason milk cartons are plastered with assertions as to the quality of the contents and information as to how it has been handled, and those who ignore them do so at their peril.
While participating in an online community is probably somewhat less likely to kill you than drinking tainted milk, it can still be risky - and as the awareness of those risks is increasing, so with it is demand for some equivalent to FDA rules.
For all the unfortunate connotations that it has picked up along the way, the term "code of conduct" has a distinct advantage: it is well-known, and folks tend to recognize what it means. Calling it a "behavior" or "be nice" policy - as we've done in the past - fails to communicate the essence of what it means, and thus succeeds only in introducing more jargon that new folks must learn in order to effectively participate here.
But about those connotations...
What is a code?
The literal meaning of "code of conduct" is "laws regarding behavior" - how a community demands that its members act. We've always had such a code - this very site is a living document as to how we expect one another to behave in various situations, from basic questions of etiquette like the use of signatures or cuss words to more complex issues such as taboos, modifying others' work or calling others out.
This hints at something important: the usual arbiter for these disputes is a group of your peers. While elected moderators and employees of Stack Overflow can and do make unilateral decisions on specific cases, these are rare in comparison to the day-to-day resolution of disputes encountered, discussed, and largely handled by folks just like you... and when that fails, folks come here to meta to hash it out, adding to that Code in the process.
This meta site, then, represents a useful guide for what one should expect when participating here; if you're ever in doubt as to how you should act, there'll almost certainly be something here to help you resolve that doubt and act in a way that is acceptable to the larger community.
However... Meta is too large to read quickly. The folks who most need this information, those who have no prior experience with these communities to draw upon, are also the least able to find and digest this vast code. This, then, is the value of a short, easy-to-read guide that covers enough of the basics to get going without drawing the ire of the rest of us.
A guide or a stick
Where folks get into trouble with these codes - ably demonstrated by some of the Linux communities you've referenced - is when they stop treating them as the law of the people, a guide to getting along... and start using them as a tool to bludgeon others with whom they disagree.
When Linus announced that code he also announced that he was taking a break to do some self-reflection and improvement... In other words, he tossed up the code as a tool that he would use to become better at working with others in his community. This is an excellent example to follow! Use the code as a mirror with which to see yourself: just as you might wipe a smudge off of your nose before leaving the house, use this code-as-a-mirror to remove any blemishes from your attitude that might cause you embarrassment or conflict before interacting with others.
But some folks in the Linux community have done something very different from what their leader has tried to demonstrate:
Some have chosen to use the Code as a stick with which to beat others, seeking out violators and working to humiliate them. Rather than helping their peers wipe the smudges from their faces, they've worked to maximize the embarrassment and conflict.
Others have looked into that mirror and, upon seeing their own marred visage reflected... They've chosen to blame the mirror. This is made worse by the actions of the first group, who've worked to ensure that no honest self-reflection goes unpunished.
Please, let's not do either of those things here!
We're all here to help
I've been thankful that, since the new Code of Conduct has rolled out, most of what I've seen has been honest self-reflection and not bludgeoning. The various meta sites are full of examples of folks looking back on their own participation and asking for assistance in improving how they interact - in short, it has been a tool for learning.
But the danger is still there: beware of anyone who pounds on the CoC like a drunken revival preacher, full of self-righteous anger and mindless of their own hypocrisy. And be careful not to be that person: remove the beam from your own eye before you say anything about the speck in someone else's. And if you're ever in doubt about how the Code applies to you or a situation you're in... Don't hesitate to ask here! The Code of Meta never stops growing...