Ok so everyone has 2 specs: main spec and off spec. You should pick one spec to roll on per run. (Ideally your main spec i.e. preferred spec) That way healers aren't rolling on tank gear etc.

I'm a druid tank but sometimes my guild has too many tanks for raids so I'll dps as cat. However, tank is my _main spec_ so I pass on every single dps piece that drops and only roll on tank gear even if I'm not actively tanking for that evening.

With that said in the original posters story we have the 2 players

Warrior: mainspec was DPS
Paladin GM: mainspec was tanking

Even though the warrior was asked to tank for the one fight his _mainspec_ is still _dps_ so he should only have rolled on dps gear. It seems like he just came in for the one fight and I assume no dps gear dropped from Thorim so he decided to grab whatever shinies he could.

The paladin was entitled to the mace unless the warrior had declared he was rolling on tank stuff and not dps gear. So the original poster did screw up unless it was agreed on prior to starting the fight that the warrior would only roll on tank loot from then on.

With that said even though I think the Paladin was robbed of the mace I don't think he handled it well at all and think he did mistreat the original poster.