...but in their defence the GPS manufacturers were apparently a little lazy, and didn't keep within their band - hence the interference.
Not in the least true. The GPS manufacturers designed their receivers, knowing that on either side of the band being used, there were frequencies allocated for satellite downlink use. The signals in these bands would have been of a strength equal to, or more likely, lower than the desired GPS signal, and so the filters were designed for this case.
Now, along comes Lightsquared, with a proposal to use these frequencies, originally allocated to satellite downlink, for something else: terrestrial transmitters. These transmitters would emit signals many orders of magnitude stronger than the signals the GPS receivers were designed to filter out.
You can't blame the GPS manufacturers for designing according to the current band plan, with the expectation that the interference environment would remain the same. After all, that's why we have band plans in the first place: to prevent interference.