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What you are purchasing is access to updates. What you do with the plugins is up to you. One site, multiple sites - it's too difficult to track such things so I don't bother. You're given a download id that is used with the "Download ID" plugin which enables automatic updates for my paid extensions on sites where it's installed. If you maintain your subscription, the download id remains active and the sites will receive updates. Let it lapse and my extensions will remain at their latest updated versions.
If you want to install it on multiple sites - go for it. I'm a pragmatist, I see it as an added value to my customers, and you get more bang for your buck.
I fixed the online/offline/away badges - thanks for the heads-up. I hadn't noticed.