Cheap filler and bottom line $. Ranchers want to produce more cattle in a shorter period and may only have about 14 months to get a calf to selling size. Growth hormones help the cattle put on weight in record time as well as reducing the amount of fat in the meat.
The hormones banned by the EU in cattle farming were estradiol, progesterone, testosterone, melengesterol acetate, trenbolone acetate, and zeranol.
Of these, the first three are endogenous hormones that are naturally produced in humans and animals, and also occur in a wide range of foods, whereas the second three are exogenous hormones that are synthetic and not naturally occurring, that mimic the behaviour of endogenous hormones
The Food and Drug Administration (FDA) current approves of five different types of hormones used in cattle and sheep. Of these, testosterone and progesterone are usually produced naturally by the animals and zeranol and trenbolone acetate are synthetically made. Zeranol mimics estradiol, Trenbolone acetate mimics testosterone, and Melengestrol acetate (MGA) mimics progesterone.
It is believed (hence the EU-US dispute) hormones are passed onto humans when they consume the foods; however, the evidence is inconclusive because of the naturally occurring hormones in these animals. Here is some scientific data concerning comparisons: http://www.blackwell-synergy.com/doi/abs/10.1111/1467-8276.00100
You can follow the US-EU dispute here. http://cofasonline.fas.usda.gov/itp/policy/hormone.html Also look up "the Texas Plan"
Good question regarding the progesterone, btw. You'd probably want to ask a good endocrinologist their take on that.
As far as the Safety Guidelines, you can contact the USDA for that information here: http://www.fsis.usda.gov/OPPDE/larc/Claims/Organic_Claims.htm