What's banned is tank-spawnkilling close by from the airport vicinity (buildings, runway, surrounding areas). This is because it's easy, resulting in repeated kills. Those are the elements: easy and repeated. And unfairly disheartening to others.
Take away the easy part, and you take away the repeated part. Personally, if I saw long range tank bombing in a DF FFA mission, I wouldn't be worried about it, unless (a) the player got repeated kills, which means (b) the setup is too easy (too close). This is why I typically don't ban *noobs* when I see them at an airport trying to camp - it's not easy for them, and they probably won't get repeated kills. But once I see the screen light up with easy kills using a tank at spawnpoint, I warn the player that he/she is taking advantage of others, and that they should stop.
Some day, Zup will find time to do what he had expressed, that is, code in something to stop advantaged tank spawnkilling. Hopefully for us, that would occur without taking away the good stuff (fun stuff like what you describe, or shooting a plane out of the air with a tank shell). We shouldn't want Zup to add new game restrictions so that exploration and goofing around isn't even fun anymore. But tank-spawnkillers (the ones that camp close by and go blam blam blam) are ruining games and making new people less interested in playing -- and they are putting the game at risk of being less fun for everyone.
Manfred