Except the unspoken rule of 2 1Handers or 1 2Hander + 1 1Hander as far as weapons go. Makes chaos very flexible and better in HtH than Imperial marines. bahhh.. just ignore my Imperial jelousy of Chaos wargear.
Except that it's not a rule, spoken or otherwise. And it never has been.
It is a rule that limits how many things you may purchase
from the wargear list
, but it is not, nor has it ever been a rule that limits how many weapons a model may carry or be equipped with.
As for BA, fluff wise BA have *always* been a very codex-adherent chapter. With the exception of a couple of special vehicles and a couple of special units, BA have always fielded straight, codex-style battle companies and units. The codex does not call for chainswords on tactical or devastator squads.
Remember, the red thirst and black rage is something the BA try to *resist*, not embrace. Tactical squads are meant to be flexable fire support, and dev squads are meant to be stationary firebases. The presence of a strong close combat option (chainswords) in either of these units would only serve as a temptation and a distraction for the brothers who are valiantly trying to resist their bloody urges. Chainswords would not help either of these squads do their job better and would only tempt them to abandon their post and leap into the fray.
It would be like giving a recovering drug addict a big jar of smack because fluff-wise, he always liked getting tweaked, even if it means he;s not doing his job right because of it.
Also note that chaos marines have the option of replacing their heavy weapons for special weapons, so that a CC-armed havoc squad is actually *good* (give them a couple flamers and meltaguns and put them in a rhino), meanwhile SM lack these options, so instead you;re dragging a bunch of heavy guns into assault where they become an expensive liability.