Referencing existing code is one thing. Copy/pasting that code, or writing new code based on it, without actually looking at the documentation to know what you're doing, is another thing entirely.
The issue is not what tends to be the correct way to properly code, but how in fact people end up coding.
It seems that many engineering decisions are made at google, to minimize the amount of damage a particular programmer can do. This suggests that while Google does/may have some very good people running the place, they also have quite a few programmers whom are mediocre at best.
These choices are about building an environment that forces a programmer to do things the right way, rather then expecting them to do such on their own.
