That's a deliberate feature Canon do as standard. The idea is the flash will fill the subject, but the long exposure will also get you the background exposed. If you don't want this mode, there is an option somewhere in the custom menu system that lets you limit it to 1/60-1/250.
On Av mode in general, I find it useful but then again I rarely use flash...
_________________Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.