I expect that they do so for the high profile apps, since the negative press from a compromised Facebook/Snapchat download would probably kill the site outright.
so to detect this manually, they'd have to be checking every single app! more likely, they've an automated system that detects new APKs signed with prominent keys.
Although it's very possible that the thing you mentioned is a fluke and somebody decided to ask the question "We should probably keep all the Facebook apps definitely-safe, so which ones are signed with this key?"