the problem is that this is a all to often one way road. You want to get employed, sign that nda right over there, no open sourcing more for you chap, cause you ll be leaking company secrets then.
That's an exceptionally unfair characterization. We're not a paranoid or asshole company that automatically equates "open sourcing" with "leaking company secrets". Most developers should know the difference here. An honest developer who might be unclear should merely check with their management. This is a non-issue for honest developers.
Obviously developers who don't understand the difference, don't ask management for clarification, and then foolishly leak proprietary intellectual property through their open source work are a problem and would not be employable.
So why should we have warmth and welcome greetings for someone singing the high-praise for the land of no return?
Perhaps at least because you're flat-out wrong about "land of no return"?
And in addition, there is legal precedent established by US court rulings for an employee's right to employment in a field of their expertise that supersedes employment contracts. Say you specialize in game engine design, you sign an NDA and non-compete with employer X and work there a while. You leave X and go to employer Y (or work for yourself, or contribute full-time to an OS project) also doing game engine design. As long as you did not actually violate X's NDA and non-compete, you can not be sued simply because you used your special skills (game engine design) at both X and Y for gainful employment. Employers can not deny employees the right to use their skills for gain.
Furthermore, if X's non-compete happened to contain wording like "you may not engage in game design for 5 years", a court would invalidate it. I know this because a former employer had wording like this in a non-compete I signed. My attorney preemptively challenged it. My former employer's attorney realized it was improper, removed that portion of the non-compete and had all its current employees sign new non-compete agreements!
If you want to win the hearts and minds of a open source comunity, encourage your trainees to spend time on our source.
I'll try.