This sounds like a bit of a rip-off, but the good news is that Google has clarified that this isn’t necessarily the case. In a statement made to Engadget, a Google spokesperson said that they do not limit third-party chargers, and that they are working with their partners to certify 10W charging for third-party wireless charging devices.
The statement reads, “We don’t limit third-party devices, in fact, we’re working with our partners in the Made for Google program to get fast 10W chargers certified for use with Pixel 3 (Belkin announced its 10W Pixel 3 charger already which will launch in the coming weeks). Pixel Stand and Pixel 3 work together through the protocol we’ve developed for fast charging. Everything else charges at the industry standard Qi 5W.”
Basically it seems that devices made under the Made for Google program should have no problems hitting 10W in terms of wireless charging speeds. However it would mean that you’d have to wait for such devices to be released, so until then maybe you can hold off on buying a wireless charger, or perhaps the Pixel Stand could be worth taking a look at.