They probably don't want to end up being replaced by an AWS/GCP/Azure service. In my opinion, the BSL is a fair license model, especially if it is limited in duration (let's say 2-3 years BSL then automatically changing to Apache/BSD).
Accomplishes the goal of preventing a cloud provider from stealing customers, but also ensures customers don't get caught in an "always tomorrow" trap when the deadline comes and the company realizes it only hurts them to fully share it.
Seems to align all interests pretty nicely.
(I'm as big of an OSS supporter as anyone, but we can't pretend we still live in a time where Google / Amazon / modern-Microsoft don't exist)
F/OSS means a very specific thing. If one can't possibly build a rocketship business, that isn't F/OSS fault. Of course, you've got an alt-movement in response to OSI and FSF's rigid adherence to its principles, speared on by tech companies who (think they) got burnt by other tech companies.
There's a lot of room between {completely F/OSS, that a cloud provider can implement and bankrupt the company supporting project development} and {completely evil closed source company like Oracle}.
In the end, we all want good software, with the maximum amount of permissions and source, free. It's just a question of tweaking the support model to get there.
IMHO, the "Don't call yourself OSS if you're not 100% OSI OSS" is counter-productive. But I understand why they do it, and I understand the history and abuses that caused them to do it. I'd just say if we no-true-Scotsman our approach in a way that precludes profitable, sustainable OSS companies... we're going to have less quality OSS. :(