Why can food be left out to cool (140F to 40F in < 6h) but not to defrost? Local government food safety guidelines for storing foods (p26) which have already been cooked stipulate food internal temperature must be brought from above 140F (serving temperature) to 70F in less than 2 hours and from 70F to 40F in less than four hours. 

Why then do the rules for defrosting (p23) (the reverse) say food should be at room temperature "for very short periods during preparation" and foods can only be defrosted in one of three ways: in refrigerator, under running water, or in the microwave provided there is no interruption in the cooking process (ie. partially in the microwave then cooked or all in microwave). If its just time and temperature, why is the cook trusted to watch the clock for cooling and not defrosting?

Previous posts have asked for guidelines and best practices; however, my question accepts the practical advise and hopes to discover the political (creating effective guidelines) and biological (does warming from 40F generate more/more harmful bacteria than cooling from >140F?) reasons. I have my own theories but what are yours?
The very document you link to clearly explains the reason:


  It is important to use methods that will allow the entire mass to thaw evenly. Any method that allows one part, for example, the outside surface, to defrost before the inner portion is not acceptable, because the portion that thaws out first will be in the danger zone before the other portion is thawed.


Bacteria grow and produce toxins while in the danger zone. If you thaw on the counter then the exterior will be crawling with them long before the interior is thawed.

The other methods are acceptable because:


Keeping the food refrigerated never permits any part of the food to enter the danger zone.
Very cold water keeps the exterior cold, which is close enough to refrigeration as long as the food will be cooked immediately.
Microwaves partially penetrate the interior and cook the entire portion very quickly; the food will be in the danger zone but for too short a time period to matter.


It's also very important to note that the cooling rules are not about simply "watching the clock". If it were that simple, they wouldn't bother with rules, because room temperature is in a pretty consistent range no matter where you are. Cooling hot, potentially-dangerous food on a countertop is every bit as inappropriate as defrosting it there.  Food service professionals will hold the food above 140° F until it is ready to serve or store, and if storing, they'll cool it very quickly by dividing into smaller portions and/or using an ice water bath or even specialized equipment (e.g. flash freezing).

There's really no practical difference here. The extra 1 hour permitted to cool from 140° F to 70° F is really only because you've just cooked it, so (a) it's been pasteurized and (b) temperatures very close to 140° F are not very hospitable for bacteria should the food become contaminated again. When defrosting from frozen, it's assumed that the food (cooked or raw) is already contaminated either from factory handling or from prior cooling in the danger zone, so you need to strictly follow the 2-hour rule.