The move directly relates to Privacy Shield and GDPR issues
A business using Gmail is by default sharing personal information (incoming email from private individuals) with a third party provider which hosts in a country with an at best questionable attitude to protecting privacy. State laws on privacy are in some parts of the US quite OK (such as California), but federal law overrides those to a degree that any government agency can grab your data without any real accountability, and as the Trump administration has more to hide than any before I suspect surveillance will only increase (there is a strong correlation between a government's desire to spy on its population and it being involved in activities that can't handle daylight).
Until now, US business had a pass for political reasons, but the legal argument was far from settled. As Privacy Shield is up for review in September, Google must create at least the appearance it is spying less on people (in the hope the EU ignores its other spying on people via Google+ buttons and statistics) because the debate will again be political, not legal. Legally they have been in deep trouble for more than a decade, the Safe Harbor and Privacy Shield agreements are legal sounding cosmetics on what is in reality a political agreement, a trade agreement.
With Max Schrems vs Facebook stripping much of the cosmetics away, the question is now if Privacy Shield II follows the tradition of any sequel (being worse than the original) by just putting a bit of new lipstick on the side that says 'Oink' in the hope of avoiding tweets from the dude with the strange hairdo in Washington, or it will be used as an opportunity to generate business for EU organisations by sticking to legal principles, also because of the GDPR coming into play next year
My money is on the lipstick.
Biting the hand that feeds IT
