The standard does not specify a specialization of std::atomic<std::string>, so the generic template <typename T> std::atomic<T> applies. 29.5 [atomics.types.generic] p1 states:
There is a generic class template atomic. The type of the template argument T shall be trivially copyable (3.9).
There is no statement that the implementation must diagnose violations of this requirement. So either (a) your use of std::atomic<std::string> invokes undefined behavior, or (b) your implementation provides std::atomic<std::string> as a conforming extension.
Looking at the MSDN page for std::atomic<T> (http://msdn.microsoft.com/en-us/library/vstudio/hh874651.aspx), it does explicitly mention the requirement that T be trivially copyable, and it does NOT say anything specific about std::atomic<std::string>. If it is an extension, it’s undocumented. My money is on undefined behavior.
Specifically, 17.6.4.8/1 applies (with thanks to Daniel Krügler for setting me straight):
In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.
std::string certainly does not meet the std::atomic<T> requirement that the template parameter T be trivially copyable, so the standard places no requirements on the implementation. As a quality of implementation issue, note that static_assert(std::is_trivially_copyable<T>::value, "std::atomic<T> requires T to be trivially copyable"); is an easy diagnostic to catch this violation.
2016-04-19 Update: I don’t know when the change happened, but VS2015 Update 2 does now diagnose std::atomic<std::string>:
error C2338: atomic requires T to be trivially copyable.