Tetra hops are hops or extracts of hops that have been treated so that the alpha acids are slightly different chemically. Tetra hops are so named because of the tetra iso alpha acids in the hops (usually the alpha acids isomerised in the same process to enable them to be added after the boil for fine bittering control). Tetra iso alpha acids do not have the group on the molecule which is effected by UV light to give the light-struck flavour effect. The big breweries use them because green or white bottles do not block UV light like brown bottles do (10% and 5% block, compared to about 90% for brown bottles). If Carlton Cold or Boags or Cascade used plain old hops for bittering, they would skunk real quick (5 minutes in the sun would do it, or several days in a fridge lit by a fluro tube). Basically, it's a means of having a product in a marketable package that has a reasonable shelf life.
These hops are used purely for bittering. There is no perceptable difference in the "harshness" of the bitterness, according to a very large amount of studies done by many different organisations. A great technological advance in hops if you ask me. There is nothing wrong with them.