This is not a bug and will not be changing. Each skill has a set of prerequisites. If a gladiator could just unlearn skills and ignore prerequisites one could for example train Total Carnage to 10, then unlearn some other skill in the tree and put those points into Hard To Hit and have both Tier 6 skills. Without prerequisites a level 1 gladiator could train in any skill right from the beginning. Prerequisites are there for many reasons ranging from balance to strategic choices. A gladiator in PoW is made up of a series of strategic choices. If you want to respec there are options available to you as some have pointed out already. If you want to speed up the process there are also options available for that too.

Good luck in the Pit!

-Nate-