I believe that the original reason why they didn't just say "you're not allowed to provide this program as a service" is that MongoDB were trying to get SSPL approved by the OSI. The OSI didn't approve it because as you mention, it's basically impossible for a cloud provider to actually attempt to comply with the SSPL. Not only would they have to release the code to all software used to make the program available, they would have to do so under the SSPL. I'm not sure why MongoDB didn't change the wording when they gave up on getting the license classified as "open source".