I am foreign to this way of working, so I can provide an outside observer perspective, with the intention to be supportive for this A2A.
Open availability means, you give priority to the company for the time period you reserved for working except for emergencies.
You say they choose only to utilize 15–20 hours of their given right of priority of your entire time of working.
This means, you can’t give any promises/priority to any other company or person or your own self for working or doing something else for this period of time.
Well, to work like this is a choice actually. You can make it or not. Maybe the company is doing some really important job that you wish to participate in under these circumstances. Maybe some day you want to become a full time employee of this company. Maybe this is not what you truly want. And what you want may change over time, too. It depends. As long as you feel happy about making this choice, as long as you make it consciously, I feel it will turn out to be beneficial. And as long as you feel that the corporation is holding the key to your happiness and not you, and they use this key selfishly and not caring about your happiness, and you submit to this and become unhappy and suffer because you aren’t expanding your consciousness to empower yourself, I feel it won’t be beneficial for you.
As to why a corporation makes this choice, it would depend on the corporation. I hope other answers who know the locale better helps you on this. I am sure, if you look at this with awareness, you can figure out a path for yourself, having developed a greater understanding of the situation. Ultimately, ‘truth’ or ‘what already is’ or ‘life as is’ is the best guide and awareness will open this up for you.