Because, as stated above, all Kindles with firmware more recent than 2.5.<something> support active content documents.
Which is why KUAL works on all devices except K1 and K2/DX with non-current firmware.
It is an active content document - try it - it works just fine on your PW-3.
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When Amazon writes: "Active Content not supported by Amazon"
It means exactly what it says.
Note that it makes no mention of the support present on **Amazon Devices** - it is only a statement of company policy, not a technical review of the device capability.
Same when Amazon writes: "Custom, after-market add-ons are not supported by Amazon".
It means exactly what it says.
Note that it does not make any statement about Mobileread supporting after-market add-ons, nor on the Kindle devices ability to run such additions.
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There are thousands of members, and an unknown number of non-members that would be very surprised to be told the things they download from this site don't actually work.