The raids I'm talking about are these little "3 times a day" raids like Tiamat and Iron Colossus. I guess it's not against etiquette to just start one of them and open the invitation to everyone, right? I seem to have pretty good luck so far with someone coming in and 1shotting them for me, and they don't seem upset about it.
Of course not. You are giving the MVP red chest away by doing this, which is the second most valuable loot chest (the host chest being the most). Early in the game it's worth losing the MVP chest to still get the host chest. In time, you'll be able to MVP/solo those raids yourself. This game thrives on raid-sharing.
Also, in the case of HARD mode raids, you are actually probably clogging the raid list of other veteran players and they may simply just be killing it out of annoyance. Just a tip, don't open low level raids (anything weaker than Omega) to Friends or you might get unfriended lol.
Leeching and helping are different lol. Leeching is when you don't really contribute to the fight and only do a minimal amount of damage to be eligible for loot to drop. For raids with up to 30 people, leeching is expected as part of the game, but it is generally never appreciated by the person hosting the raid (exception: you put 9 ranks of Treasure Hunter on the boss then peaced out). For raids of 6-18 people, leeching is considered rude. And in the case of some 6 player raids, even 1 person leeching can cause the raid to be unwinnable.
Joining a raid hosted by someone else and actually fighting is not leeching. Even if you're too weak to solo or MVP the fight, you can contribute by casting useful buffs, debuffs, heals, etc.
MVP stealing is also part of the game. If you don't want someone else to join your raid and take MVP from you, don't call for help from the public (at least until you've done enough damage to where it's impossible for someone else to overtake your score). Of course, pre-arranged raids with friends/guild/etc. can be a different matter where people stronger than you can hold back to respect MVP agreements.