Burzum's third album - the one ever present in "top 10 black metal albums ever" lists - uses a rock beat, rock drumming and casual rock-style guitar strumming throughout almost the entire album. No one has any qualms about this. Why then is rock in 'folk metal' a problem?
First, you assume that rock invented anything. Rock and blues and jazz invented nothing. They are styles.
Next, you forget that half the album is ambient, and while it does have its metal aspects, these do not terminate in a rock-like feeling; they are used as a gateway to something else.
Other bands simply drop into the rock and roll and make a clumsy hybrid and call it a day.
Later Graveland is, as one poster pointed out, uneven. I'm finding the newer stuff to be much improved over "Awakening," which has a rock rhythmic sensibility through its guitar work and doesn't, like Burzum, channel it elsewhere. It's a Lord Wind album adapted to Motley Crue.
Newer Graveland is much improved. The two Lord Wind albums may be his post-"Following the Voice of Blood" peak.