Blizzard has now removed the ability to disable experience gains for Level 70 characters.
In a recent hotfix, level 70 characters can no longer disable their experience gains in Retail WoW.
This change was unannounced, but it seems that this is intended considering the Support Article for Disabling Experience Gain was updated 4 hours and players have disabled their experience gains in Early Access.
The War Within scaling seems very off compared to previous expansion, where level 70s are completely overpowered compared to the content they are facing. If you played MoP Remix, its very similar to a level 10 solo'ing a whole dungeon and in War Within, a level 70 character could ultimately solo Normal dungeons. Many players were starting to realize this and leveling characters in 3 hours which progressed into farming other content on 70s rather than 80s, which then naturally lead to people turning their experience off on level 70s. Now… you still get to benefits for only a limited amount of time per character. Use it wisely!