Tri bikes are for athletes who can't ride, sort of.
The riding position is different to a road or TT position, and is indeed illegal in UCI events. The rider adops a fairly relaxed touring style, then rotates forward to achieve a flat back. The bike is made with a steep seat-tube, a short top tube and stable steering geometry.
This position is useful for people who havent spent their teenage years growing into a hunchback time trialist, and favours the muscle groups that are well developed in runners. The tri bars help the athlete rest his/her arms and shoulders after the swimming section.