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.