I am wondering if your point is that only students who work hard can achieve high scores or if your point is about policies that don't allow a perfect score. The above posters have dealt with the first possibility in depth so I'm going to address the second.
I do know of teachers who have policies against A's or 100 %, it being that nothing is perfect. However I disagree with these policies because I don't think giving 100% is necessarily saying that an assignment is perfect. I think if we are working on a 100 point scale then the best work that does all of the things asked of the student, and does them well, even if it is not perfect, deserves a 100. I know this policy is particulary popular in terms of not giving high grades early in the school year, under the assumption that those grades can only be earned over time. I also disagree with this as what you are assessing at any given moment changes. I don't expect a student in the first month of school to be doing the same quality work as one in the last month of the school year. I am grading based on the assignment and the expectation at that time and place. I'm not going to say you can't get an A, I am going to say that A level work gets harder over the course of the year and over the course of each consecutive school year.