> I'm also surprised that simplejson is used

2.6's json lacks simplejson's C accelerators [0] which makes it roughly 20 times slower than simplejson (2.7's simplejson is also ~50% slower than simplejson, as new performance optimizations were added in 2.1.0, as well as memoizations). Simplejson is also updated more often, which lead to it having less bugs, and interesting new features (the ability to natively serialize decimals was added in 2.1 for instance)

simplejson should be the preferred import, with json as a fallback if available.

[0] 2.6's json is simplejson 1.9; 2.7's is 2.0.9; the latest release of simplejson is 2.1.3 and 2.1.4 is in preparation