It's interesting to see the responses to this article and contrast with the responses to the article regarding Facebook's possible 5B fine. I don't know the motivation behind this decision but I can't help but see the two sides of the same coin.
If we decide as a society that third-party apps misusing the platform in a way that hurts the users or otherwise looks bad to the outside world is the platform's fault, even if this was done with the user's consent (which the app that harvested data for Cambridge Analytica had) and furthermore decide that such conduct deserves a disproportionate fine (which 5B is, given that much larger breaches without any consent have generally gone unpunished) because the platform as a whole is owned by a large successful company, we cannot also expect large tech companies to keep supporting open platforms that allow third-parties to thrive. The economic benefits of giving control to the user are fairly marginal and theoretical, while the risks are extremely large and potentially existential.
I'm not intimately familiar with this particular API, but it's almost certain that it can be abused by third party apps in a way that makes look Google bad. Platform openness has its advocates and detractors at every company. In light of what's going on, it would be hard for the advocates to win any argument.