It's not because the OSI prefers, it's because it's the mainstream, generally accepted current definition of the word. I mean, if I sold you a "broascasting system", would you accept something that casts seeds out?

The term "open source" itself should not imply a specific license

It doesn't. It implies a set of conditions they have to meet. BSD, MIT, GPL, Apache, WTFPL, are all different Open Source licenses.