My thought process was, the Specialists Building mechanic you use to build the colony from scratch at the start intended for you to have them take the first step. Later on, when the Colonists arrive, this mechanic changes.
Before, you can just plunk down any of the (not greyed out) buildings, but you need to manually assign a Specialist to deal with it. After, Colonists, usually Carriers, gather the materials from storage and then begin construction. With just one guy (mayyyyyybe 2) and whichever tools they happen to have in their inventory. You can assign Specialists at any time during the gathering and construction phase, and they will either assist with gathering resources, wander around until all resources have been squared away (accounting for material in transit), or get to work on construction of the building.
The reason I think it is an exploit is, the devs probably intended for Colonists to take over building construction (after the player tells them where to build of course) when they finally show up - which gets better with better tools, but the Specialists building mechanic remains. Combining the sole Builder and a Specialists accelerates the process nominally, and stacks the more Specialists you add.
I should note exploits often involve intended mechanics. One famous exploit is found in many Mario games - jump on the same koopa shell a specific way, and you can max out your lives. All from the mechanics of koopa shells.
The Specialist Building mechanic can be abused because you could probably build a well in seconds with 1 builder and 10 specialists. And wells take a long time to build.
But we can agree to disagree.