Clarity of meaning, regardless of sentence length, drives Standard Edited American English.  For other kinds of English that stress sentence brevity, clarity is much more easily achieved, and related punctuation rules are simpler.  For example, in the interest of space (which equals money in a print medium), journalists often omit the comma before a coordinating conjunction joining two complete sentences.  True run-ons don't exist in the English of journalism, although comma splices and fused sentences are both unacceptable.