You'll notice that, while doing quests for the Telvanni, that they're more focused on internal power struggles and personal agendas than they are, say, on profit and expansion (Hlaalu) or honour (Redoran). They even choose to forego using the Morag Tong to settle disputes, which is the traditional means of doing so. As far as they're concerned, the Telvanni still left alive at the end of the conflict
must be right, or he would not have succeeded... kind of like social Darwinism, actually.
The Telvanni are probably the least traditional of the Great Houses. Even the Hlaalu must retain some semblance of tradition in order to keep people happy, but the Telvanni do not seem to concern themselves with any kind of morality or conduct at all, believing that it inhibits progress. "Telvanni think that wisdom confers power, and power confers right."
The Telvanni do not place much value in the traditionalist Dunmer moral system, and so that taboo against Necromancy doesn't concern them.
As for the rituals, that's never specified in-game.