Stop thinking like an engineer and start thinking like a lawyer: this, like everything else in your contract and HR handbook, is ultimately negotiable. I mean, heck, my day job put it on the table while trying to prevent me from leaving, and I was a Japanese salaryman. (They had a couple of offers -- another one involved more work and less pay.)
Any particular company may be more committed to full time employment than a Japanese megacorp, but I strongly doubt (and have personal knowledge that it is not the case) that all companies are totally committed to full-time only.
Just out of curiosity, what was the reasoning behind that? Was it something that could have ended up being a good position, or was it a hand-wavy type thing?
I had more compelling options.
No employer is going to let you work part time. There's a lot of time and expense involved in finding and maintaining programmers, and they'd rather have a full-timer instead.
As for starting your own company, those always take more time than a full-time job if you want to make any money.