Massimo,
Let me give a go at your questions too.
Can you use proprietary software without any limits? Yes. Well, disregarding the limits of the software itself. I cannot use a sound editor to edit videos.
Can you study the source code of a proprietary software? Sometimes. Some proprietary vendors do release their source code. Like,
Microsoft. In fact, seeing source code does not make a program non-proprietary. There are open source proprietary packages. Look at the code, study it, but you cannot release modified versions.
Can you modify the proprietary software? Nope. However, neither can I modify open source software. I am not a programmer, therefore theoretical ability to modify, say, my favorite looper Kluppe is quite irrelevant to me. I tried asking developers for help - nobody could do it. Period. For me virtually zero difference. Kluppe might as well had been proprietary.
Now, to be fair, because it was open source and free, Falk could write a patch for it to support JACK2. So small changes are sometimes a benefit. But for larger changes? Almost no difference.
Can you redistribute proprietary software? Sometimes I can. Depends on the license. Mostly I cannot. I also don't think that I need to redistribute it. I can tell you that FL Studio is a wonderful DAW. You can try it out, if you like it - you can pay money for it and work with it. If you have no money, I don't think that it is my mission to get you FL Studio. I might want to help you and buy a license for you, if I want to help. But I don't think you or anyone else is entitled to a free copy.