Might also be a question of tracking a "conflict of interest", where the professional in question might work on a similar proprietary product inside the company.
IBM certainly allows you to do your side gigs, but you need to be transparent with the company about what it is and that there is no conflict with your work.
Unless proprietary tech is used, I think I would advocate for laws protecting people from being restricted working on projects that provides such a public good... but that is another argument entirely.
When it comes to open source contributions, both business conduct guidelines and open source mandatory yearly certs are unambiguous about transparency and approval of open source contributions. So it's pretty mandatory I think to inform, and limited if any impetus not to open open about it.
Working for a big corporation is a bit cyberpunky/Snowcrashy in that way I suppose.
IBM allows open source work. But there are rules, particularly about transparency. Just consider the sco lawsuit that won't die, and you may get an inkling why. This person appears to be in the open source / linux part of ibm. IBMers all must accept open source contribution guidelines. It sounds like this person simply didn't fully follow. Managers response may have been heavy handed but again none of us here knows details unless somebody provides them.
I notice there's four other ibm addresses so there is a story there.
Disclaimer and disclosure - IBMer who contributed openly and with management awareness and never got this kind of stern instruction :-/