I agree. As long as you quit before you launch or sell something. One of the best ways to start your own business is to learn everything you can under someone else, then go work in that industry.
As an employer myself, I encourage and seek out employees with this mindset. If they are truly entrepreneurial and want to own something, that's good for me if they apply that mindset to my company (and are remunerated accordingly, of course.)
What I meant (and what the Amazon issue is about), is that it's not ethical to start announcing or selling a competing product while you work there. In the case of software, it's seriously uncool to develop competing software in your "off time" and then come out of the gate with it.
No. Your employer is simply irrelevant outside work hours. It's as if you had quit.
Did you have a salaried job in programming where that was in your contract?
Yes, this is absolutely OK.
The experience, knowledge, and connections you gain by working for a particular employer belong to you, not the employer.