Historically: yes.
Now: no.
> possibly criminal
Sans some sort of formal agreement (which platforms like HackerOne might facilitate), it's definitely criminal. (IMO at least not unethical, to be clear.)
Again, sans some sort of contract either one-off or platform based. If SEGA wanted a prosecution, they would almost certainly be able to convince a prosecutor to press charges. The prosecutor would certainly get a guilty verdict. (Or, much more likely, a guilty plea with a bit of prison time and stiff probation.)
This still happens from time to time in much more ambiguous situations. E.g., https://www.nytimes.com/2021/10/15/us/missouri-st-louis-post...
Fortunately, there's a bit of a gentleman's detente among reasonable white hats and reasonable companies. But if you venture much outside of the small set of companies who rely on and have technologists in senior leadership, the story changes fast.