As explained on the manual, it's because all these airports already have a customization made by us, so the parking editor won't work there.
For Aerosoft EDDM, we made a quick-n-dirty customization, just indicating where all the jetways are, which usually cover the most annoying problem of seeing all stairs on a gate, but Fly-Tampa Athens is fully customized, and we are doing ( still not finished, though ) a full-blown customization with 100% precise vehicle placement for UK2000 EGLL, there's really no need to customized these ones.
You can check which airports have been customized by GSX, by looking at the fsdreamteam\Couatl\GSX\airports folder, the file names should be self-explanatory. Yes, you can remove our customization by deleting a file, but keep in mind it will restored automatically with the first Live Update.