From this perspective, it becomes obvious that the question, "Why is the speed of light one unit length per unit time?" is an absurd one. There is no other alternative.
What Cushman means is a number like Pi is always 3.14159265, to us or to hypothetical aliens. Whereas the speed of light can be just effectively represented as 299,792,458 or 1 depending on the units you use.
What you are effectively asking about ck2 is calculation of dimensionless physical constants which are of ratios of like-dimensioned physical constants, for example the fine structure constant. These could in theory be calculated one day without using experimentally derived values . . . maybe.
The wikipedia page on the physical constants has a good run down on this: http://en.wikipedia.org/wiki/Physical_constant#Dimensional_a...
As does: http://en.wikipedia.org/wiki/Fundamental_physical_constant
It does measure something we can quantify in units that make intuitive sense: it is the smallest possible distance per the smallest possible time. c is not arbitrarily that; it is the fact that ties those two dimensions into a single universe, without which they would not have anything to do with each other, and without which the concepts of time and distance individually wouldn't mean anything to us.
Is that a little clearer? I'll agree it's not possible to really grok this stuff without at least doing a bunch of reading on Wikipedia.
Unless you're counting in base Pi, then Pi is 10.
Pi is only 3.14... in base 10. Just like the c is ~2.99 * 10^ 6 m/s because we define a meter that way.
And what do you mean "spacetime doesn't necessarily have to obey"?
This isn't to say that, given the tenets of simulationism, we can't hypothesize an "external time", and describe the rate of our universe as some amount of Tx/t. But the fact that that's an amount of time per time should also demonstrate why it's a meaningless concept in-universe.
The question you think he's asking is: why is there a limit to how fast light can go? For this we have a reasonably good explanation; it's more or less a consequence of basic geometry once you've understood relativity.
The question I think he's asking is: given that we have a value of c why is it that value of c and not some other? For this we have no damn explanation, it's just an arbitrary constant that got baked into the universe for some reason. Perhaps one day we'll understand it in terms of something else, but perhaps it's just as in-principle unanswerable as "so why the fuck is there a universe in the first place?"