During PBA national and regional stops I've been to, the only courtesy given during practice is making sure you dont kick the person next to you.
There is no clear/written rule about giving lane courtesy during practice for any league and tournament.
But based on this situation, in my opinion, you are correct. It is also a little out of courtesy to others on your pair also to throw one ball at a rack and not strike, then to throw another ball on the strike line intentionally not to make the spare. If the person did in front of me did that, I would ask them "whats the point? I thought you were shooting at the spare." The humorous side of me would have hit the reset button when I saw them shooting at another full rack as their ball is going down the lane.