According to Jagoex at Warlock Therapy, this was done to stop a practice of artificially inflating your Personal Rating. Our own Zach Yonzon explained this smurfing concept. Players would form teams with their main and alts, winning with their mains and losing with their alts, thus ensuring a slow and steady Personal Rating gain. Now that the equivalent Team Rating is required, those people working the system have to play the Arenas the way Blizzard intended.
Tyren also mentioned that though the new purchasing requirements are in effect as of today, they won't be displayed by the in-game vendors yet, but in an "upcoming patch." So though you won't see it listed in-game, the Team Rating is now required.
EDIT: Clarified last sentence.