It's not a morally legitimate reason, but as others have pointed out, SE's guidelines are (intentionally) vague, so it's not really an action someone can get into trouble for. I do suspect, though, that vote-kicking more frequently targets healers who twiddle their thumbs for half a dungeon because they are loath to cast any offensive magicks.
This is just lazy. If you have downtime and MP to spare, then cast some offensive magic. Otherwise, you're simply giving healers a bad name.
It's important to distinguish between 'ideal MMO design' and what is currently present in FFXIV. I think healing should be intensive enough that it's a full-time job, but that's not how FFXIV is designed. The average dungeon run doesn't require anything close to a healer's full attention (obviously there are exceptions, which healers should adapt to), and healer damage doesn't strain the ability of a tank to hold hate. Given these facts, saying "healers should only heal" in the context of FFXIV is patently absurd.
This isn't a 'debate' between two competing philosophies with equally valid positions. It's a divide between a group of players who remember that they're playing FFXIV, and a group of players who want to ignore that fact and cling to principle.
This isn't helping matters. There's a difference between not letting DPS die (i.e. focusing as much on healing as need be, up to 100% of the time if your gear / skill / party requires it) and taking a hard-line position to never DD. I've had some groups as a White Mage where the DD and tank can largely go through a regular pull without more than a Regen spell or two thrown their way; are you seriously going to support a healer who looks at that situation and says 'awesome, I get to watch Netflix while running the dungeon'?
The poster you quoted should absolutely be shamed. Trying to cram every situation into a rigid philosophy (one that, coincidentally, gives them a giant free pass for lazy dungeon running) practically begs for such an outcome.