What result is most likely has no impact on fairness if both players are subject to that same likelihood. For example, if both players used a set of loaded dice that rolled 6s half of the time, the game would still be fair - different, but fair. The standard dice roll more 1s. Using dice that roll fewer 1s gives an advantage over the other player.
The game might be designed to work with 1/6 probability of each result, but the dice used to play give a less even spread distributed more on the 1. Altering the spread using different dice (whether they have a small bit of lead near the 1 edge or have a different shape - which, outside the level of pathetic-ness, are really the same thing) is cheating.
No kidding. That is why people who buy 'better' (which you just admitted) dice buy themselves an advantage.
I am not sure what your argument is (which brushes are used certainly does not make one space marine more likely to kill another), but casino dice are an advantage over others player, everyone knows they are an advantage, and the people who use them to gain an advantage might as well be moving an extra inch or giving models free upgrades.