Entirely different worlds.

People think "The Federal Government" is a monolith, and they could not be more mistaken. It is a collection of individual entities, operating largely independently and often at odds with one another. 18F is a different entity with a different set of goals from the law enforcement (FBI/DEA/etc.) and intelligence (NSA/CIA/GeoInt) agencies.

In any case, it's also different parts of the tech stack - encrypting everything over the wire with HTTPS is fine so long as the underlying encryption is weak and/or the NSA have a set of keys. The FBI can always just issue a national security letter to the party with whom you are communicating.