Ender Crystals can grief even if gamerule mobGriefing is set to false.

Endercrystals are no mobs !?

Well people can use 3rd party programs to spawn them, they are entities I know. Well, they interact with the Ender Dragon. If this is invalid, close it. TNT carts have to do block damage because they are TNT carts, they have TNT.

Yeah, that's like saying TNT shouldn't make explosions in mobGriefing is turned off.

Ender Crystals have absolutely no AI, therefore they aren't mobs. They are still entities, but they cannot do anything without interaction.

They will not "Willingly" explode unless you make them.

Then hoping Mojang makes the entityGriefing gamerule xD (TNT, TNT carts and Ender Crystals wont grief, but they will deal explosion damage to the player/mobs).

So why is this marked now as "Works as intended as other entities like fireballs, tnt... don't destroy blocks when they explode!?

Confirmed for 14w30c

tnt does destroy, a fireball is from a ghast who is a mob a eder crystal isn't a mob nor from a mob, that's why it is intended

I'm a co-owner of a 1.11 vanilla server, we have a no grief rule, we disabled TNT but cannot find how to stop people from using these to grief. Is there a place this can be found? Or is it just going to grief our server either way?

Rogger R Rabbit, ender crystals that are spawned on the towers have a ShowBottom tag to show the bedrock stand at their base and the ender crystals that are used to respawn the dragon have to be placed on bedrock so so if you want to stop griefing you could run a few commands in repeating command blocks in this order:
1) /scoreboard players tag @e[type=ender_crystal] add bottom {ShowBottom:1b}

2) Optional, if you want the players to get the ender crystal item back:
1) /scoreboard objectives add placeCrystal stat.useItem.minecraft.end_crystal
2) /execute @e[type=ender_crystal,tag=!bottom] ~ ~ ~ detect ~ ~-1 ~ obsidian * give @a[score_placeCrystal_min=1] end_crystal
3) /scoreboard players reset @a[score_placeCrystal_min=1] placeCrystal

3) /execute @e[type=ender_crystal,tag=!bottom] ~ ~ ~ detect ~ ~-1 ~ obsidian * kill @e[type=ender_crystal,tag=!bottom,r=1]