The 2001 USA PATRIOT Act provides the US government with the right to search/seize data held by any US company or its subsidiaries. It does not matter where the data is physically stored, if it is held by a US company (Microsoft, Apple, Google, DropBox, Amazon...) then the US government can seize the data. However, in order to do so it is necessary for the US government to obtain a court order for the purpose of an anti-terrorism investigation - they can't just idly decide to grab your data.

Note that these rights are not terribly different to the rights of other countries to access data (see Hogan Lovells' white paper).

Further information: The Act:
  • 2001 "Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism" (USA PATRIOT) Act (Link).