Sometimes I want to click the pen tool on the end of the path. This can be to change the direction of the path (if I'm doing text along a path). Or it can be to extend the path.
Most the time this is no problem. But sometimes Illustrator decides that no, it's not the pen tool I want. The program will change the pen tool to the subtract anchor point tool and delete the point.
Is there a way I can avoid or disable this unhelpful feature?
Make sure you are actually selecting an endpoint, not a midpoint. Clicking a midpoint will remove the placed point, clicking an endpoint should not.
Another thing to check if your pen tool is acting weird is to make sure none of your keys are stuck or have a cat sitting on them. For instance holding
alt would change the pen tool's behavior as well.