Smoking is legal too. Something being legal isn't in-and-of-itself a compelling reason to do it.
That's questionable. While they may not know, it could be considered fraud since you are knowingly rolling back your JB in order to hide the policy violation. That said many state consumer protection laws would require Apple to prove a link between the JB and the damage before they could legally void your warranty.
This is a good point, but it diminishes with each iOS release. Also many of these tweaks are buggy or interfere with existing functionality. For example many of Activators gestures conflict with the UI of apps.
You may not like it, but your mistaken. Read your contract.
For the record, I do JB my iPhone; but the suggestion that it has no downside is simply wrong. At a minimum JB requires longer waits between official releases, frequently reduces performance/stability and makes exorcising warranty claims more difficult.
I'm currently sporting 4.3 and may not JB it again. Tethering via wifi was the last compelling reason for me to JB.