Because that's its accuracy for knowing your absolute position in a short period of time. If you use it to determine relative position over a long period of time, it's much more accurate. Apparently there's something called "Carrier phase tracking" which has an accuracy of 2mm for surveying. Or they could have augmented it with ground-based transmitters.
Still, 4mm is quite a small distance to measure with GPS, even with a 2mm accuracy mode.
Blocking access to the memory space of other processes has been a solved problem since timesharing in the '60s and '70s, right?
I assume they aren't running in a flat address space with no MMU, so maybe the problem is that the apps all operate under the same user ID, which bypasses the usual multi-user protections. Perhaps "run each app with a unique user ID" will be something we'll see a lot of in the next few years, like the no-execute bit and ASLR were in the 2Ks?
I don't know how "very early" in the '80s you're talking about, because I remember being intrigued by the appearance of a strange new channel that played music videos. MTV started in mid-1981. And we had a big city cable company, not "two guys with a spool of RG-6". (At the time it was UA Cablevision; later the franchise was sold to Rogers and then Time Warner.)
The early '80s were the glory days of C-band TVRO satellite. Maybe it was just that your cable company at first only had access to channels which just happened to mostly be commercial free. Or maybe you watched a lot of C-SPAN.
Premium (as in pay $5 or $10 or more per month) movie channels run movies all the way through. The rest of them are just networks without OTA broadcasters. And "early" cable-only channels were just that, experiments like on the internet in the early 2Ks.
I refuse to get pay TV, and now that I've upgraded my DSL to Uverse without TV, the phone company has switched from half a dozen mailings a month to get me to switch to Uverse to half a dozen mailings a month for me to add pay TV. The thing is, I was amused to see that qubo was only in the highest of their three tiers, because in the next market over, it's being broadcast on an OTA channel.
I don't know when and where it was that you had "ad free" cable TV, but cable TV was originally (in the '70s and earlier in the US) for people who didn't want to put up an antenna and mess with it to get a good picture for local channels (with the commercials intact). Then around 1980 or so, my family got hooked up to a cable TV system that wasn't just an antenna redistributor, and had cable-only channels. I was surprised to find that most of the channels had commercials. So at least in the US, ads on cable-only TV channels date as far back as 1980.
Ironically, you now usually get a better picture for OTA channels by NOT getting cable, because they can re-compress the signal to a lower bandwidth. So that cuts down the original reason to get cable.