Ban for ninja

hi i have new character 2days fresh dk and now i have ban for ninjalooting . i roll only in group loot for ms (need) .. its mistake or ?

No mistake. You are allowed to roll need only on direct upgrade for your spec in dungeon, like crit+haste for dps, defence+dodge+parry+resilience for tanks (yes, in TBC tank items have resilience).
So if you rolled on tank item due to it having several points more strength than your gear, it is absolutely justified ban for ninja.

and if im only plate there i cant roll need deff ?when they greed for dissenchant ?

Ofc you can't. If you doubt it, go read loot rules for RDF. And yes, there is reason for all of it. No, it won't be changed. If you want it, go and communicate with your group, instead of being greedy. By the tume you get enough gear to "tank", that gear that you got meanwhile will be outdated.

You press NEED only for upgrades for your current spec. Everything else is GREED, no "but no one needs", "but my actual main-spec", "I'm the only one", etc, it's not going to work.

You can always talk to people, if all of them agree, you can NEED whatever. But silence does not mean they agree, lots of people have gotten 5 days for assuming.

If you want some specific gear item, you make your own group where everyone is informed about who gets what. I highly suggest you make yourself a community, a guild to play with likeminded people. It will be great, that's how this game was built to play.