There can't have been that many end users who had Windows 10 Pro and went into group policy to turn off the store. So you're looking at small businesses who were using PCs with Win10 Pro on them (likely that came with the PC) that were turning off access to the store but can't any more. The IT admins for these companies are the people Microsoft wants to upsell.
Lets say that there are 500 businesses who care about this feature each with an average of about 20 PCs (probably a high estimate for PCs, low estimate for number of businesses). That's 10000 PCs that Microsoft could potentially convince to upgrade, at (a quick guess based on Google) $120/PC, to Win10 Enterprise, or a potential $1.2M more in revenue that doesn't cannibalize one of their other businesses (assuming more changes to differentiate Pro vs Enterprise). Probably the people who will upgrade are people with factory computers running Win10 Pro.
And for people that don't upgrade, they get to promote their app store. Win-win.