It seems silly in today's world where anyone can roll a custom SoC and have it spun out by any number of Fabs over the weekend, but 30 years ago custom chips were pipe-dreams for all but the most advanced tech firms. The $2 microcontroller you use to turn your lamp on has more processing power than a desktop computer from the early 80s.
By putting artificial constraints on off-the-shelf GPS technology, it forced bad actors away from off-the-shelf technology and instead forced them to roll their own. This often meant an inferior product with potential compatibility issues and significant development time and costs.
The same thing is done with export controls. It seems silly today but in 2000 they were limiting where a Playstation 2 could be exported because at the time a beowulf cluster of PS2s would have been a powerful super computer for a country like Iran.
\* Missing words...
\\ The next generation of GPS doesn't have Selective Availability, presumably because today there's 3 alternatives to GPS out side of US control.