My guess is that the public channel was a moderated channel with additional rules to which you agreed by participating. Alternatively, when you open a private conversation with someone it is not moderated. Therefore, people might curse, swear, and be otherwise as offensive as they like; so long as what is said doesn’t contravene the EULA, such as a real life threat or illegal activity.
You should bear in mind that many people consider in game chat, and EVE-Mail to be an element of PvP gameplay. Sometimes people deliberately engage in offensive behaviour to provoke you. It’s part of the experience. I’ve done it before, and I’ve had the same done to me. Just don’t take it personally - and if you don’t like it, then block them.
Personally, I have no problem with calling people poopy-heads, idiots, fools, or otherwise folks who don’t know what a “good fight” looks like. I’ve not problem with calling people out for being dirty blobing cunts… too cowardly to face me in solo PvP. Yes, I can be downright rude sometimes, but, my tongue is a weapon.
Sometimes I can get a response, and at other times, a well timed and provocative EVE Mail can provoke an opponent into an unforced error. It’s all part of the game.