Earlier quoted context omitted.
I thought SA was removed because we discovered ways around it. Specifically differential GPS(tl;dr is that if you have a known point on the ground you can take a measurement there and get the SA offset, this is widely used to compensate for atmospheric error in GPS).
Although WAAS and other similar techniques can be used to improve the accuracy there are many impracticalities of using secondary sources. The reason stated was to encourage civilian uses of the better accuracy. The other federal departments agreed there was more to be gained from civilian use than lost by making the more accurate signal than lost but limiting its use.
Actually, at the time, once other potential user groups saw how beneficial the technology could be for them (airlines, farmers, on and on) these groups threatened to send up their own GPS systems unless the government agreed to enable full accuracy. The government faced with losing control entirely or meeting the demands of industry chose two work with industry as the US legal writ did not extend to space.