Ever since Windows 10 was first announced, we've been hearing about "Windows as a service", and Windows 10 being "the last version of Windows". Microsoft is taking updating Windows seriously, making it difficult to stop or delay updates to Windows 10 machines, especially ones running Windows 10 Home.
That inability to control updates apparently also includes app updates from the Windows Store, as noticed by the folks over at Windows Supersite, and addressed in a Microsoft Answers thread on the subject. The issue is that for Windows 10 Home users, the setting to "Update apps automatically" is grayed out in the On position, meaning there's no way to deny or delay app updates. The setting notifies users to "Contact your system administrator about changing this setting", a fairly meaningless request for single Windows Home device.
Some of the complaints on the Microsoft Answers thread point out that in areas where Internet service is capped by monthly usage, updating apps in the background can lead to some unexpected data usage, although to be honest that's going to be a problem with the Windows as a service model in general.
It also isn't clear if the grayed out button is a "bug" or a "feature", or if Windows 10 Home users will have more control in the future. A moderator on the Microsoft Answers thread, "Smittychat", suggests that there may be changes to the update process coming in September.
Windows Update notes that "(u)pdates won't download over a metered connection (where charges may apply)", but that probably doesn't apply to data capped services. Of course there are other reasons for not wanting apps to update, from buggier new builds, to features lost in new editions, to general FUD over anything occurring on a user's machine without explicit consent.
With Windows adapting a service model, updates are going to be part of that service, and the old days of running XP and never updating seem to be apparently over.