[fbender]

The Launch Abort System (LAS) is heavy, and since it requires Dragon V2, which includes more systems only required for human missions, the overall system mass is even higher than "just" the LAS. Heavy is extremely expensive, ergo it does not make sense economically to equip the cargo version with the LAS. Simply said, the chance of failure multiplied with the cost of the cargo is less than the extra cost an LAS on every mission would mean.

[david-given]

Although, given that rocket launch failure is a fact of life anyway, then putting the LAS on unmanned payloads means you get a much higher chance of testing it for real. Which means you have a much higher chance of knowing whether it'll work when you are in the Dragon it's attached to, which I reckon is a good thing.

Also, I thought the whole point of the Dragon v2 launch escape system was it uses the engines and fuel already in Dragon for orbital manoeuvring and landing; even the unmanned version is going to be using that. So it shouldn't be that heavy.

[fbender]

Because of the LAS, Dravon v2 has to carry more fuel than v1. And it's not only the rocket motors that are heavy, but the supporting equipment and structure (peak loads are different for some parts of the structure). All of that leads to higher mass. The v2 is not just a v1 + LAS rocket motors, but in large parts a severly altered design.

As for having a chance to test the systems: That's what the recent pad abort test (and subsequent in-flight tests) are for. Plus, the engines, being liquid engines, are each tested prior to installing them on the spacecraft. You'll need to do that anyway, and you are always hoping that you'll never need to use that system (does not fully apply to Dragon v2 since they intend to use the LAS system for landing the capsule with the rockets some time in the future).

I still say it does not make sense to equip a cargo mission with LAS capability. (Disclosure: I'm an aerospace engineering student, M.S., so it's an educated guess rather than a theory on my part)

Sidenote: Solid LAS systems, as used on most other capsule systems, cannot be tested like liquid systems. You'll need a comparative test with built-alike motors but cannot test the same system that will be installed in the spacecraft.