I don't think that for a Seanchan woman, torture is necessary and either way I doubt any Sul'dam would actually call it torture in any case. When we see the interactions in Falme when Mat is trying to free Teslyn, the Sul'dam working to break her says "it is a hard way to break a Damane, and it is a shame to break them down so completely, but when trained that way, they always became the meekest Damane." When we see the Sul'dam breaking the Aes Sedai that is TRULY what they are having to do. As much squabbling as we routinely see from the Aes Sedai, there is no doubt that they are some really hardcore women. Just the test for the shawl alone entitles them to being called badasses, and they hold the top tier of power in Randland, bowing to nobody except their own titular head the Amyrlin. It must be incredibly difficult to break them, but like anyone, if you push hard enough they will break, and never be the same.

In training a Seanchan-born Damane, however, I agree with many of the previous comments about the social conditioning regarding damane helping to ease the transition for the newly-collared. When acceptance to a particular fate, sucky as it is, coincides with a strict social norm, it is much easier to accept. When accepting the fate of being treated as dogs, directed everywhere they went, being confined in small spaces, having punishment used on them at every turn (and extreme punishment at that... Egwene up to her neck in boiling water, yikes!) The most powerful women in the "world" would find a harsh transition indeed.

I think it has been viewed as torture because of the extreme measures needed to bring Aes Sedai to heel, combined with the direct violation to Randland social norms thousands of years old.