Slings have a range of 18" and S3 you can "double shot" at under 9" with S2. You can move and shoot (but not double shoot).
Light Crossbows have a range of 18" and S3. They can move and shoot.
Shorbows also have a range of 18" and S3 and can move and shoot. They can also use the Massed Missiles rule.
Since in most circumstances the shortbow or sling is equal to or better thna the light crossbow why does it cost +1pt to upgrade sling armed skirmishers in the above list, but an upgrade to short bow is for free? Should it be that Light Crossbows are also free or even -1pt?