In the past, fixes have shown that they are not fans of people using previous tiers over current or far into current tiers. Most likely, the 2P would have been basically required for large combustions and maintaining high DPS, so they nerfed the tier for people to regear into the next one. Same thing happened to Arcane from T15 -> T16, and other sets in other tiers.
I have gotten tons of complaints about Fire. It is a shame, but understandable from Blizzard's end. They really should have just switched the bonuses from T17 to T18 and vice versa, but they most likely didn't have plans in place for what the T18 bonus was when they designed T17.