My primary guess is that it's due to that fractional turn bit; being able to switch in and out of tactics mode without much trouble is probably too easy to abuse. It'd be kinda' silly to be able to fire three shots, take a step back to break LoS, swap to clock for a second, then back into tactics to do it again, to give an example.
You automatically switch back to clock mode when you change levels, which makes using it in multi-level dungeons (mines, spaceships, etc.) a bit less annoying. A bit.
I guess the ideal would be for it to check if there's anything in LoS + some amount of space, to keep the player from abusing blast weapons around a corner or something, plus that fractional pause bit you mentioned, so time catches back up to movement. It is more than a lil'annoying that you can wipe a level clean of inhabitants but still have to drag your party turn by turn to the nearest staircase.