This particular case has nothing to do with etiquette.
The Events as OP told it.
1. He shows up as the first person of his group to kill Dreadtooth.
2. Another group is killing him in Instance 1.
3. He moves to Instance 2.
4. The rest of his group takes their sweet time getting there.
5. The other group switches to Instance 2 and kills him again.
6. OP logs on to the SWTOR Forums to cry about something working as intended.
It would be another thing if the other group did stuff to wipe the OP's group or the like. Like I said in a previous post, you need to be fast to get World Bosses, it is part of the nature of the beast.
I would bet that the other group did not even see the OP's group as they were not even close to gathered. SWTOR has had people that have been surprisingly cool about world bosses, I have been in several Nightmare Pilgrim groups that took turns with other raids until one killed him. I played FFXI back in the day and guilds were absolutely cutthroat about world bosses. I remember a Japanese guild had a man hold a world boss that could be CCed for 14 hours to get it's daily spawn time to Japan-time. Training mobs into rival groups camping a World Boss was par for the course, especially after you lost a pull.
it is a etiquette issue as a group was there getting ready to kill him and the other group NEEDED to wait its turn. i can understand why you dont know this as you are part of the problem having never learned this old world boss rule. it is exactly like i said a popular mmo never taught its player base etiquette and now games must be designed around it. your ffxi example is a prime example of the lack of etiquette. just because it was the norm there and in wow doesnt mean it is the "norm". this is how guilds get black listed and have problems recruiting. a majority of people dont want to play with a bunch of richards.