The description of the sniper turret says it has an ammo consumption multiplier of 30 which should mean it eats 3 whole magazines per shot, but it only seems to be consuming 1. Is this intentional?

This is correct. Factorio's engine will only allow a turret to consume at most 1 full magazine per shot. The main reason is for modded magazines.

Consider the case where a mod adds a magazine that has substantially more durability (shots) than vanilla. Consider that the sniper turret's damage multiplier is naturally very high (20x). If I just did the bare minimum of 10x ammo consumption, then these sorts of modded bullets could become unintentionally much more powerful/efficient with my mod. This 30x multiplier prevents this scenario with modded magazines, while ensuring there is effectively no difference between 30x and 10x ammo consumption for vanilla.

Some calculations. Measuring total damage output for one magazine in a sniper turret vs a gun turret (before tech bonuses):
-Vanilla mags => x2 total damage
-Modded mag with 11 durability (eg modded mag is just very slightly more durable)
-----10x ammo consumption => x3.63 total damage
-----30x ammo consumption => x1.81 total damage
-Modded mag with 30 durability
-----10x ammo consumption => x2 total damage
-----30x ammo consumption => x0.66 total damage
-Modded mag with 31 durability
-----10x ammo consumption => x2.58 total damage
-----30x ammo consumption => x1.29 total damage
It isn't a perfect system of balance, but the numbers I have are intended to give some amount of futureproofing.