Check validity of signature values r,s,v

Applies the following statements from the etherpad:
9. Transaction strictness

Transactions are INVALID if:

v < 27
v > 28
r >= secp256k1n = 115792089237316195423570985008687907852837564279074904382605163141518161494337
s >= secp256k1p = 2**256 - 2**32 - 977

Also applies to proc ecrecover
Why is `v` not checked inside `isValid()`?