It occurs to me that using a tank to attack someone who is not in a tank is a spectacular example of the very definition of bad sportsmanship.
Perhaps that's why the tank can get you a Bad Sport rating so quickly - it's completely unbalanced and so can blow away (pun intended) any timers that might be in play by accumulating Bad Sport 'points' faster than they can dissipate.
There's no difference between using a tank or sticky bombs. The system is broken anyways. Sometimes you blow up three cars - bad sport. Sometimes it's 30 or 40. It's just a part of the trashy game you're playing, and will continue to play and so will everyone else. So you get used to it.