I think they need to be graded on a curve because one SAT/ACT test administration could be more difficult than another. If you scored the test strictly on a percentage correct basis, then had colleges compare kids who took the tougher test with kids who took an easier one, the comparison is unfair.

Grading on a curve for these tests doesn't mean the same thing as grading a math class test on a curve. In fact, to get an 800 on the SAT math you need to get all 54 questions correct. The math curve is harsh - my eldest got two wrong and skipped one question - that earned her a 730. When she took the ACT the math curve was better - two wrong out of 60 questions and that was a 35 (36 is the highest score). The reading curve was harsher - one wrong out of 40 was a 34 (could not get a 35 on that ACT test).