I expect they use it because it's cheap.
The danger with trans fats is that they are not found in nature but when the body digests them, they get put into cell walls etc in the place that a "natural" fat would go. The body mistakes them for fats that they already know about, sort of analagous to kids who used to drink yellow dishwashing liquid that was in virtually identical containers to lemon squash, then rushed to hospital (I think that packaging has been banned or at least discouraged).
It's thought that trans fats, as well as artery and heart problems, are a major contributor to macular degeneration of the retina, major cause of blindness in the first world, due to the wrong fats becoming incorporated into the retina.