Giving power to the users can be costly and so often software is proprietary if only to mitigate risk. For example, I work for a satellite ISP. If the software running on the modems were completely open down to the radio firmware, normal customers could bring down the entire network by commanding the radio to setup a continuous wave that interferes with other people in the same satellite beam. Or a competitor could look at our source and see which TCP optimizations we have made for the satellite link and copy them (depending on your stance on software patents, this would be hard to protect).

So yes, you can "buy" and "own" one of our modems, but solving the aforementioned issues would take a lot of thought, a lot of money, and probably a lot of (hardware) cryptography to prevent malicious radio usage if the software were completely free as in freedom.