Yes, they are. Being famous makes them a target for theft or even simple vandalism. I recall seeing a locally-famous tree in Ecuador, large for its species but not that rare, with burn marks all up the side from local vandals. I can only imagine that a truly rare, large, or old tree would be even more prone to damage.
Also, now that we have that out of the way, I’d like to suggest a very potentially dangerous piece of information about endangered species: recipes!
Maybe not if you’re majoring in chemistry.
Or, how delicious they taste sautéed with a little olive oil and garlic. 
Recipes.
Quick aside of possible relevance: archeological sites get the same treatment for the same reasons. My environmental consulting company does a fair bit of archeology work, too, and hard copies of our reports are frequently submitted with the location maps redacted to discourage artifact collectors. (The documents are often done for public agencies and are thus accessible to the general public.) The client gets them on CD only.
The concern over the largest trees isn’t just strictly related to mindless vandalism. Haven’t you ever been so upset at your significant other that the most sensible thing to do was to cast a ritual spell by poisoning a famous tree? Sure, we all have. But I can see how some people wouldn’t like it. People who have never been IN LOVE.
There was a famous bright yellow pine tree - very unique - on the Queen Charlotte Islands of BC. Some nutbar chopped it down to protest logging or some such weird reason. Never underestimate the ability of humans to be destructive and stupid at the same time.
The book The Wild Trees talks about the search for Hyperion, the tallest so-far-discovered tree, in Redwood National Park. It talks about vandalism (initials carving, cutting off souvenirs, etc.) towards other significant trees and the deliberate secrecy imposed to prevent that with their find.
And it also talks about how difficult it is to actually measure the height of a tree, especially in a situation like Redwood National Park where the terrain itself is highly variable, the forest floor isn’t always soil (covered by a thick layer of duff and debris), incredibly thick foliage obscures line of sight to treetops (for both visual confirmation and laser rangefinding), etc. To really accurately measure a tree, they had to shoot a fishing line up to some mid-level branch, pull up a climbing rope, manually ascend the tree to the top, and then drop down a measuring tape.
Even given GPS coordinates, finding the “correct” tree is no easy task. They all look similar from the ground. Here’s another guy’s journal of his search.
It was a Sitka spruce with a genetic mutation that caused it to look golden. The guy was a logger for years and then cut down the tree to protest logging, or something. All you can really say about him was that he was nuts.
Interesting… and I’ve learned something about how hard it is to find trees in a forest, too! 
Sometimes it’s difficult to see the trees (or one particular tree) for the forest.