Thanks for replies everyone :) The only one where you can actually SEE the heating element is the tank. The others look just like regular buckets and you can't even see that it's heated. Yes, we washed out the buckets before filling them with drinking water. Here are the buckets I'm talking about
The "tub" which is inside the big stall with my mare: API 16HB Heated Bucket, 16 gal. - 2170851 | Tractor Supply Company
The buckets which we use for the minis: API 20FB Heated Flatback Bucket, 5 gal. - 2409323 | Tractor Supply Company
And then the big tank with the heating element at the bottom, which I couldn't find a photo of. It's like the big black/grey trough you see in most horse pastures.
We empty and clean out the tanks regularly. I even left one tub unplugged to see if they might drink it then, and they didn't. I'm thoroughly confused! I'm ESPECIALLY confused why the BO went and bought three more huge heated tubs, even though the horses aren't drinking out of them...