Is this 'defect' still found in 2012 Xperia? If it is, then after my contract ends, I won't take Sony phones anymore. What a waste...
It's not a defect, but a design to lock you into using their own earpiece. Easy way to get pass it is to buy a Sony earpiece made for Xperia series. TDK also has a specific series for Xperia.
Fortunately it appears that they wake up their idea already for the newer Xperias, so rest assure your new Xperia won't have such a problem.