But what we generally do is, you know, not run plugins that phone home :).
Edit: I just looked on a Linux machine - things have changed a little. That used to be my go-to document for iptables, which is why I referred you to it. You probably want to consult something more up-to-date.
In any case: iptables can do filtering based on who created the packets.
Even later edit: it probably goes without saying, but just in case -- this solves only half the problem, because it can only identify the process based on its PID. This makes it pretty trivial to overcome PID-based rules. The usual solution, of course, was to filter based on UID and run the program under a separate account.
I don't know if this got solved in the meantime, I haven't used iptables in a while now.
Why? That's the case for most major proprietary UI apps -- open source versions are usually subpar in several ways (not because of some inherent issue with open source as a license, but because of time constraints, unglamorous work, not enough UX/UI volunteers, no top/down vision, etc). Where they do get the upper hand is usually in offering more configurability.
>Even more why Objective Development themselves did not try to port their very successful product to other platforms.
For Windows it might make sense, but which other platforms would they port to? Linux desktop users are few and far between (1/5 to 1/10 of Mac users), and there's not much of a paid software market.