I'm sure that it depends on the school, but yes, Waldorf is supposed to get tougher as the student gets older. Steiner divided childhood into 3 seven year periods, and each period is supposed to be more intensive. Just because he didn't think that children under 7 should read doesn't mean he wanted illiterate 21 year olds... the whole 21 year system was supposed to be very rigorous. I seem to remember that, though it seems a very modern trend, Steiner himself advocated a lot of internships, because his system is very much based on "learning by doing."
It's possible that since a full Steiner education was designed to go until the child is 21, schools feel the need to cram high school full of 8 years worth of work to make sure that the child's education is complete. I don't know enough about that to know if there is any basis in fact to that theory.
There are probably practical modern reasons, too. Most Waldorf schools are expensive private schools, and parents expect their children to get into a good college after they've paid a down-payment worth of tuition bills over the years. In order to retain their top students and get them into good colleges, even the Waldorf schools have to compete with other prep schools whose children are doing internships and a lot of homework.
Trying to live a simple life in a messy house in a complicated world with : DH, DD (b. 07/07), DS (b. 02/09), and DD (b. 10/10)