The description of the [Fly](http://www.d20pfsrd.com/magic/all-spells/f/fly) spell reads:

> 
> The subject of a fly spell can charge but not run, and it cannot carry
>  aloft more weight than its maximum load, plus any armor it wears.
> 
> 
> 

It is however difficult to figure how a character can charge without running.

What is the rationale behind this?