You've always needed to be able to implement APIs in order to have compatibility, so in essence, this decision puts into case-law what what already assumed to be the case.
Let's say a software vendor out there is making a ton of money with a product that exposes a certain API.
Let's say I know a way to implement that API in such a manner that its performance will be improved by an order of magnitude. (without using any patented method in my implementation)
Can I put together that implementation and market it as a competitive product?
In my experience, APIs of commercial products are disclosed under the terms of their licensing agreement. So, one could argue that I cannot market a competitive implementation, if the license terms I agreed to (to gain knowledge of the API itself) prevent me to do so.
Assuming a case where there's no contract involved, patents are typically what would cause trouble. The original implementor will typically have a patent on some silly thing their software does, and you have to do that thing to make a compatible implementation, so they sue you for violating their patents.
To go back to my question, there are a lot of entrenched players out there who rely on an age-old user base to keep afloat. If you can provide a better implementation without asking them to rebuild their client applications, their customers will have an easy road to migrate towards your implementation.