First, you are muddying things because it’s not a product called OracleJDK vs a product called OpenJDK, it’s “Oracle’s download called (the) JDK” vs “a different download that can only legally be called OpenJDK.” More on that below.
It’s also more confusing than you indicate because you are excluding all historical context. The “regular” JDK everyone installed up until Java 8 was the “JDK downloaded from Oracle” and the open source OpenJDK was, for lack of better words, a knock-off with poor compatibility with a lot of enterprise software but used because you couldn’t include JDK in your distro’s package manager (even though it was completely free to use) because the license wasn’t GPL compatible. So everyone that cared about vendor-approved compatibility downloaded “the JDK” from Oracle as a tarball and used that. Those that were running home software could get away with using “apt install openjdk” instead.
After Java 8, Oracle pulled a huge switcheroo and said everyone should use OpenJDK instead, and their licensed customers could use the Oracle-provided JDK instead. They didn’t put it behind a paywall or even login, and left it available for free.
So if you missed the bit about OpenJDK being officially blessed to use or you continued to just go to Oracle.com to download “the JDK” for commercial use, you were now in violation. Some say Oracle licensing is never confusing by accident and the official recommendation to switch to a previously inferior alternative wasn’t a charitable move.
If you don’t know OpenJDK is what you want, you search for “JDK” and the only entity allowed to use the plain “JDK” moniker is Oracle. Even now, just Googling for “download JDK” gives you Oracle’s JDK (that’s twenty years of PageRank and inbound links pointing to what was once a “free” download - there’s no outranking that!) as the first result.
(Given Google’s IP history with Oracle, they should show “Did you mean OpenJDK?” and include those results too :D)