There is some confusion over exactly what Qi charging accessories can go above the 5w charging speeds. Apple currently sells Qi charging pads from
Belkin and
Mophie. According to both of those manufacturers,
only companies that partner with Apple (like them) will be able to be certified and have their Qi chargers take advantage of the 7.5w power output.
7.5w is an officially supported mode for Qi chargers by the Wireless Power Consortium but it appears
Apple is using hardware identifiers to restrict access to accessories that it vets.
As to why Apple is being so restrictive, only they really know but their marketing for the high-speed wireless charging feature on the Mophie and Belkin Apple Store product pages may yield the answer.
Apple writes that they use ‘Qi wireless technology to deliver safe, quick-charging speeds’.
The implication is that other Qi chargers on the market are not safe enough to deliver wireless power above 5 watts. Apple has previously
come under fire for the dangers posed by fake Apple chargers, so there is some precedent for them to be cautious here.
Apple may quickly approve many more Qi chargers to enable the 7.5w speeds on more than two products, after iOS 11.2 comes out.
Both Belkin and Mophie firmly believe only their accessories will support 7.5w on iPhones at launch.