The only reason I would imagine you would hear that is because of the current state of the LFG system.
There are a LOT of DKs out there, and some of them are rather good. Many of them are not terribly skilled or experienced, but they can climb fast and easy to 80 because of the huge head start.
DKs have all the necessary tools and abilities to be fantastic tanks, as hearty as any other class (some would argue more in certain situations). In order for that to happen though (and this is true of any class), they need to take the right tools and use them well.
I imagine that feeling is simply a matter of a lot of less skilled or simply not tank geared DKs who are playing as tanks so they can get into groups. DPS DK friends of mine have said they can wait 30-40 minutes to get a group as DPS, but tanks get a group the moment the join the queue.
So, generally as a class? Not hard to heal. I wouldn't be surprised if there were a higher incidence of people tanking when they just weren't ready for it though.