Why is everybody suddenly on good terms with Grom Hellscream?

Solution 1:

Grom redeemed himself when he refused to drink the demon blood a second time, with intent to never be enslaved. When doing so he moved to attack Gul'dan outright physically. Though he was handled easily by Gul'dan this turned him into at least opposing the same enemies as us heroes in the story. There are quite a few communities debating if he deserved this redemption story and to what degree this was glazed over by Blizzard.

Most useful 2 references: http://wowwiki.wikia.com/wiki/Grommash_Hellscream and http://www.mmo-champion.com/threads/1806309-Should-Grom-be-forgiven-(Spoilers)