Why isn't Z system source code public?

While the tutorials shed a bit of light on how systems are written, the Z systems seem to be quite a bit more sophisticated. Having access to their source code would allow people to step up their game in writing their own alterations.

It is kind of hard to imagine Zorro saying to the people:
"Here, take this rapier to fight the rich, but I'll have to blindfold you while you're at it!"

I think one reason stated for the Z system developer not to reveal the codes is because someone can always use the code to do reverse orders. Example, someone can just sell you orders and that someone will make money.

Hmm, but if the Z systems are optimized and proven to be profitable with a high probability, then we actually want to execute those orders no matter what - even more, if someone is willing to provide the liquidity for us every time we trade, even if it is profitable for them, it would work to our advantage, too!

I think if one can find a system with slight edge of a system, it will be considered blessing. I am not very sure if a high probability (>60%) will ever be available to retail traders like us. Maybe HFT?

By high probability do you mean the percentage of won trades?
That percentage is not too important, since even the Z systems only get around 50% right.
What matters is the amount won or lost per trade, where the Z systems achieve pretty nice results.
That is why it would be very interesting for us footfolk to be able to look at the guts of Z systems and learn advanced techniques for our own projects.

All the Z systems are based on published algorithms, so you can check how they work. The reason why we can not include the actual source code is simple, and explained in the manual: To prevent fore-running. Otherwise the systems would expire quickly and we had to change the code all the time.

Would it work for you to publish an older version of the Z-Systems, mainly to answer questions like that

Yeah! Older versions would be great to see.
Not even so much for the actual entry and exit conditions, but more so for seeing a completely finished script with all properties like portfolio management etc in gory detail (and not just a snippet about it from the manual but production level quality).

The Z5 system is obsolete. It would be an example that would not hurt publishing and yet zorro users would get a good insight of how a complete script looks like.

We do not really have old versions. Z5 is still in test run with other currency pairs, for maybe being reanimated in a future version. But the Z strategies do not look very different than for example the strategy in workshop 6, so you can use that one as a template.