So you know, a warn is, "Hey man, so you know you have to at least attempt the objective, so make sure you start moving with enough time to complete it."

Dropping the bomb is not illegal, as long as he has enough time to complete the objective before the round ends. This isn't gun game--you can use 100% of the time in the round even as the last person (you tend to piss people off doing so) but as long as you get the objective completed in that time frame it is fine.

Telling someone what the rules are is completely fine. Try playing on the WCS server without any of the rules enforced and you'll learn how much of a shit show that can be. The rules are there to make the game run smoother and keep the community healthy.

Also, as for your later argument about taking his fun away, go talk to Zero about that. The rules are the rules, and only he can change them. Personally, I agree with the current objective rules. It allows for some camping, but means that eventually there will be some action. Otherwise, you'll end up with 2 people that camp the entire map and there would be an absurd amount of downtime for other players.