This is against law regulations in many countries (i.e. Holland). Why then? What is the reason behind?
I can, somehow, understand cornfields, lakes etc., because you can easily avoid those, when you have your head open. But, leading a biker against traffic on a one-way streets may easily get him into a serious problems. And isn't that easy to be avoided.
Is there any reasonable explanation, why it was implemented (not removed yet) in Google Maps?
Here is an example blog article, that the same happens when using bikepaths in London.
Best Answer
Because you need to know which streets are one way only.
Mapping is rather expensive, and the layout may change without warning, so it may go out of date quickly. Also, the mapper may have incorrectly noted down a street as two way for bikes, since many streets are indeed.
Of course, Google could throw in more resources and polish this, but there is no real need, as cyclist have eyes and can see that it is a wrong direction. If you are still online, you can take the next turn and let it recompute the route for you.