C-Span's position is best defended as a matter of general principle: when a court case is being discussed, it is best (if possible) to allow both sides to speak, even if one side has lost (i.e., Irving's), even if the losing side deserves to have lost, and even if it deserves execration.
In this instance, I don't believe their position is defensible. The record of the trial speaks for itself - and certainly with more truth than Irving can be counted on to provide. Not to mention that if Irving's alleged correspondence on this matter can be believed, he's already had the opportunity to present his "side" to the C-SPAN audience. And, it would seem, C-SPAN certainly had no interest in "balance" at that time!