This is the crucial difference between the AGPL and the GPL: the AGPL requires you to make the code available to users for whom you run the code, as well as users you give the program to.
But, what I don't know is -- is there any other AGPL code that minio doesn't own, but that was otherwise contributed to minio? Because, presumably, they aren't actually giving their customers the minio program with an AGPL license, rather they have whatever their enterprise license agreement is. If this is the case, and there is AGPL code that's not owned by Minio, I can foresee problems in the future.
Additionally, in order to CHANGE the license, if others contributed code under that license, you would need their permission, on top of the fact, you cannot retroactively revoke the license for previous versions.
I can't see how this is a defensible position for Minio, but I'm not sure they really care that much at this point.
That said, if there was contributed AGPL code, they couldn't change the license on that part of the code w/o a CLA. AGPL also doesn't necessarily mean you have to make the code publicly available, just available to those that you give the program to (I'm assuming AGPL is like the GPL in this regard).
So, that I'd be curious about it is -- (1) is there any contributed AGPL code in the current version? (2) what license is granted to customers of the enterprise version?
Minio can completely use whatever license they want for their code. But, if there was contributed code w/o a CLA, then I'm not sure how a commercial/enterprise license would play with contriubuted AGPL code. It would be an interesting question to find out.