Dot visability is 10k, meaning you can see a dot once they are with in 10k of you.
On the map friendly dots are shown always, even those below dar (500 above ground level), my guess, is because then you know "where to fight is". if you see a bunch of friendlies heading to a base you might want to tag along. Makes it easy for people to "join in the fun" Enemy dots are only shown while inside an active radar circle, as long as they are above 500.
Bar dar is the bars that are at the top of each sector on the map showing the enemy strenght in the air of that sector. The bigger the bar the more planes.
The "cheating" your talking about isn't cheating, its called spying ( you are alowed to talk about spying) While not "rampant", spying does happen, and is frowned upon as much as it is in real life, but some people will do ANYTHING to win.
Once you make one run and are "caught" either by one guy, or spotted on dar, there are those that will then watch for a small dar bar to pop up in out lieing sectors. Knowing a bomber guy is online, its easy enough to keep an eye out for them, and as buffs are easier kills than most fighters, you'll draw a lot of attention.