Why not? There are plenty of services out there that officially support one ecosystem while ignoring the others, in spite of the fact that they're making it harder for themselves by doing so. If we were to stick to the idea that they have to register somewhere before those services could be added to the app drawer then we'd never see some services supported. The ability for a user to add those themselves is a massive bonus for Chrome OS as it takes that out of the hands of the companies and gives it to the users.
Imagine all the grandmothers being given a Chrome OS machine who know that they go to the app drawer for most things they do, but have to remember to go to the bookmark bar for that one other thing they want to do. How confusing would that be for them as they advance in years? Meanwhile, their family members can set up this and have everything they use in one app drawer for them. That massive disturbance in the force that you're feeling is a billion old women crying out "Ooo, isn't that clever!" all at once.
As for being confusing for users, most people who use this assume you have to be online for most things. Those few that are able to get into flags and enable it are likely able to understand Chrome OS to the degree that they can manage this and explain it to others.