The link is dead now, but this is the published minutes from an ABC rules committee when they were discussing linear strikes to the knee:
2. Discussion of Linear Strikes to the Knee Joint
Based on a letter from Nelson "Doc" Hamilton, the committee discussed this issue which seemed to garner attention due to UFC 97 and its Anderson Silva-Thales Leites fight. Profato opened by stating that he thought linear strikes to the knee joint should be banned and Mazzulli agreed. Foster stated that contestants skill levels are progressing annually and that we need to protect the contestants.
Lembo suggested that this maneuver be banned in the amateurs, but allowed in the pro ranks. Lembo stated that this type of kick is difficult to land and is able to be defended by checking or movement. Lembo stated that kicks to the side of the knee joint seem to cause worse injuries. Lembo added that we allow strikes to the head and eyes which could cause worse injuries.
Dr. Wulkan stated that from a purely medical standpoint, kicks to the side of the knee joint while the leg is planted are more dangerous and could cause worse injuries. Dr. Wulkan stated that the leg usually must be straight, not bent to increase more serious injury risk from linear knee joint strikes. Dr. Wulkan stated that other types of kicks can be thrown with more power and cause more damage.
Erickson asked what combative sports disallow this maneuver. It was noted that these kicks are not allowed in kick boxing but that is due to the fact that kicks below the waist are not allowed.
Kizer stated that there are ways to defend this offensive technique which occurs directly in front of the competitor. Kizer read from a statement from a NAC consulting orthopedic physician who stated that he did not see the need to ban this technique and saw other allowable techniques which could be more damaging from an injury standpoint.
Foster stated that Anderson Silva is an anomaly. Foster stated there is not enough data on this rarely utilized technique. Kizer reminded that each Commission can require more restrictive rules if desired and Foster stressed the need for uniformity. The Committee unanimously agreed that it be suggested that linear strikes to the knee joint be banned in the amateurs but remain unchanged at this point in time in the pros.
Two doctors, one an orthopedist, said it wasn't especially dangerous.
It also doesn't seem to have a history of insane knee destruction in striking arts where it's legal.