Its also totally illogical that Fox's laser goes only about 5 feet in the Demo. But I'm pretty darn certain that's a change that's going to be in the final version (to appease someone's shouts of cheap!) so I'm also pretty darn certain that logic has nothing to do with their attempts to balance the game and hence that Samus' projectile game has in fact been nerfed beyond repair. Sorry : (.
From a purely physical-reality standpoint, both cases are illogical. But speaking from the idea of the game itself.
1. Fox doesn't need his lasers; he really doesn't. All of Fox's combos, killer moves, etc. are close range. At least Falco could've used his to stop projectiles, screw up recovery and set people up via SHB into killer combos. Just like he wrote on the dojo:
"Run! Fly! Move boldly! For such moves are the essence of Fox!"
Fox is meant to be a speedy close-range fighter, so to that end he's designed the Blaster so that it has to be used at a closer range. Not just that, forcing people to remain closer with Fox overall makes Fox a more active fighter (after all, the closer you are to your opponent the more you have to move and do something).
2. Shortening Fox's blaster solves the issues of:
-Laser camping
-Blasting somebody once and running around all over the place while the timer runs out, or otherwise just getting in silly pops and then forcing your opponents to come to you.
But let's look at Samus' Missiles (as a Samus player, I don't have much qualms with her Charged Shot. I'll take the less power for a faster charge and a faster flying shot, just as long as its stun and knockback direction remain the same). What problems does making her Missiles worse, and THAT worse, solve? What does it do for Samus, who is supposed to be a long-range fighter anyway?
Samus' Missiles are weaker. Okay, that's fine, 'cause they were pretty cheap in Melee. But apparently they're so weak that they do virtually nothing. Where is the sense in that?
And what little they do is apparently vertical now. But what good is vertical knockback if it's so weak? There is simply nothing, absolutely nothing, sensible about what Samus' Missiles have become. A slight power drop? That's reasonable. But not this.
Edit: (I'll be honest; I think Samus' Missiles aren't as nerfed as we've thought.)