It seems it was the intent of
CASSANDRA-6238 originally, though I've tracked to the commit of CASSANDRA-6309 (f7efaffadace3e344eeb4a1384fa72c73d8422b0 to be precise) but in any case, ConsistencyLevel.validateForWrite does not reject LOCAL_QUORUM when SimpleStrategy is used anymore, yet ConsistencyLevel.blockFor definitively cast the strategy to NTS for LOCAL_QUORUM (in localQuorumFor() to be precise). Which results in a ClassCastException as reported by https://datastax-oss.atlassian.net/browse/JAVA-241.
Note that while we're at it, I tend to agree with Aleksey comment on
CASSANDRA-6238, why not make EACH_QUORUM == QUORUM for SimpleStrategy too?