I imagine that Steve went with the speed of light times 5 seconds (1,498,962,290 meters??) as the maximum possible range for 'beams' since it seemed the most logical thing to do at the time.
Be that as it may, game balance has changed significantly since then.
Possibly for a few reasons:
1. Weapon balance
Not any more. Beam ranges are negligible compared to missile ranges, and even to theoretical beam weapon ranges.
3. Complexity of tracking a beam over multiple increments - ignoring any diffusion (how/would a beam of high energy dissipate over distance in space/vacuum?)
That should be modeled by the falloff in laser damage, not by anything to do with the fire control. If we're speaking of lasers, a beam slowly spreads out due to diffraction. For a given laser with a fixed focus, it's like a pair of truncated cones with the focal point at the smallest diameter. Sort of like ><. If the laser is variable-focus, then the focal point can move. The size of the beam at the focal point is directly proportional to range, and nothing else. The laser can be thought of as being a cone, with the size set by the mirror and wavelength. The power of the laser has nothing to do with it. I hope that's clear enough for you.
As for other weapons, it's more complicated. Particle beams spread out as they travel, so their path is actually a cone.
I am sure you could come up with some TN based techobabble to explain how the laser is sent as a pulse contained in a bubble of wibbly-wobbly-timey-wimey stuff.
Sensor packets come in that way.
Or, you could explain it as ... your beam fire control range is actually the maximum range that you can open a micro-wormhole to shoot the laser through. A nanosecond wormhole just big enough to fire the laser through is generate in the laser's 'barrel' with a terminus in a direct line to the target. You can obviously still miss.
Interesting. That's actually quite good, although it does leave the question of why lasers theoretical range is so much longer than their practical range.