Interesting. I wonder if perhaps the carboys are the culprit in this case? I have heard of some poor quality carboys coming from Mexico.
On the other hand I am inclined to agree that use of these handles on full carboys scares me a little. I have done it in the past with never a problem and none of my carboys exhibit the fractures that you show, but I pretty much exclusively use the Brewhauler straps now when moving full carboys. I feel much better using them, and the straps also provide a bit of a cushion if you are setting the carboy down on concrete. Still have the handles installed to but use only when carboys are empty.
To my recollection there was never any instruction or warning that came with the handles as to their proper use.