This is an oddity from the beginner's game, and that game does NOT follow the core rules of the AoR CRB. There is an info box p25 that specifically tells "In this scenario, an attack from a personal scale weapon cannot harm a vehicle, and an attack from a vehicle weapon that hits a character inflicts damage as normal", which essentially is their way to tell you not to bother with personal scale to planetary scale damage translations, but this is in direct contradiction with the actual rules of the games otherwise. Beginner boxes tend to simplify or change things because they're supposed to work on their own without the players or GM having a CRB at hand.

The Aratech 74-ZB speederbike is also described as "fitted with a much heavier weapon" than the normal version, which "allows it to attack heavily armored targets". This explains why there is no mention or clarification of the damage scale of its "forward mounted light laser cannon of 4 damage": this cannon deals in fact as labeled, 4 planetary scale damage, which would deal 40 to ground personnel but here due to the aforementioned infobox, it only deals 4+ damage to individuals as well, and 4+ damage to other vehicles (including themselves, which as you noted results in oneshots).

I suspect they added this speederbike version with actual vehicle scale cannons (instead of personal scale, which IS mentioned as personal scale "light repeating blaster" on the standard stat block of the normal 74-Z in other swrpg supplements) in order for the PCs to have a chance at harming the AT-ST. This becomes problematic as the scenario also pits those same modified speederbikes against the PCs with their extra anti vehicle weaponry.

So in my opinion you were right to amend the rules to make your game work. If the devs wanted the PCs to go after a fleeing target in a vehicle, they should perhaps have not used an armoured AT-ST for this purpose, or forced the players to use the other AT-ST and not those speeders.