The honeymoon is over
"there are no current plans to extend it to other Office 365 services"
No current plans, so it says.
"we're being responsive to our customers who have requested this capability", adding that "organizations can rely on their own internal policies, procedures and communications to ensure that those individuals making self-service purchases are complying with company policies"
Users often demand things outside of the way things are done, ignoring situations in which things are done a certain way due to silly little things like regulatory compliance or statutory requirement. While user input is not always ignored or rejected out-right, sometimes they have to be told "no" until a later time. In any case, Microsoft knows damn well that users will violate policy if it suits their convenience. The three precipitants of user misbehavior are a motivated user, an attractive feature, and lack of hard prevention on the part of the administrators. The first two parts are already present, and Microsoft is prying custodianship away from administrators.
"'Microsoft will provide standard support for self-service purchasers.' In practice, though, users may well channel queries to their internal IT support and this is something Microsoft cannot control."
Neither does Microsoft care to control how users get support. Users will demand in the most obnoxious ways their organizational IT support provide assistance with their surreptitiously purchased product, and they will get it the same way iPhone users eventually get support in an Android-only shop -- to shut them up.
"Admins may choose to assign a centrally purchased license to users of the cancelled subscription."
Translation: once your users become accustomed to their new processes under your IT know-it-all noses, then train new hires and the work-flow becomes dependent upon these products, even when stymied by the short-comings, when the department manager who bought the crap in the first place leaves and takes the license with them, YOU will get to foot the bill on a new, central license for a product you never approved, never wanted, and probably already purchased an alternative product in the first place. (Was there a place for a period in that sentence?!)
In environments where administrators do not have necessary controls, many due to management requirements, users will happily bring in elicit licenses from home or improper licensing from school for a business to use. Then managers ring up IT wanting to know why suddenly they cannot get some program working, and they need it working right now because of a deadline tomorrow.
"Currently the company is in effect stating that it knows better than IT admins what is best for their users. That this move is unpopular with those admins is unsurprising."
Microsoft does not give a flying flip about the IT admin in the field. Even with the love-fest of the early 2000s with all the free TS2, inexpensive TechNet and Action Packs, it was all a ploy to make us feel good about being on the ground floor of an IT workplace revolution and turn us into support drones making enough money from it to keep us happy. But Microsoft no longer needs us as the users can plug in directly to Microsoft's systems management and product licensing and, having done our jobs, we can bugger off.
This is like being the guy at school whom all the other guys want as a friend, but what they really want is to bang your sister.