Didn't Bricklink make the Studio software prior to Lego's 2019 acquisition? That doesn't completely justify Lego, but I think it is another bit of relevant nuance to the situation.
It is a very common thing to do. Startups cut corners and don't get attention cause they don't have a lot of money. But get bought out by a multi-billion dollar company and it gets noticed.
Folks here seem to be contacting the bricklink team already for clarification, it will be interesting to see how this plays out.
Feels to me like open sourcing would be a win for them, provided they don't link to any other code containing proprietary licenses.
Unless Bricklink actively hid it during acquisition it doesn't justify Lego at all.
https://www.lego.com/en-us/service/help/apps_video_games_dev...