Maybe it works but why someone would want to use 3rd party service with no privacy policy, TOS, owners etc.

Because all you give them is a (usually) public book and your kindle email. Most of us aren't afraid of anything that can be done with that information.

There's a reason most states in the US have laws protecting library records. I don't know who runs this site or what privacy laws and policies (if any) they operate under, so I don't know how they might feel about certain books I might send through it and what they might do about it.

I don't want to send a copy of "And the Band Played On" (for example) and start getting Chick Tracts.

To do that you need to disable airplane mode on your kindle which means you have to see ads :(

I’ve had Kindles with ads. They are almost unnoticeable. If you are tight on your budget it is not unreasonable to choose the ad-supported model. Often the screen saver ads are offers for books that match your interests.

Or you could give up the subsidy Amazon provided by seeing ads and then ads are gone permanently.