Only part of your statement is true. Yes, I agree there is no hardware change, no service center appointment require for them to enable AP for potential Tesla owner. They can flip a switch, click on a button on a browser, etc.
But I wouldn't necessary call it pure profit, because the AP software doesn't come out of thin air. They have to hire engineers to develop that software, and continue to put those said engineers on their payroll so they can improve on it.
We all love the software updates from Tesla, but they costs money.