Your question seems to contain an assumption that there's some inherent ordering of programs from "easy" to "hard" (law and business being "easy" and math being "hard"), and that a letter for an "easier" program would automatically be stronger. There's no such ordering.
Any reasonable professor would try to write a letter that addresses the strengths and weaknesses of the student in skills that are relevant to the program in question. Different programs and areas of study require different skills, and the strength of the letter will depend on the student's strength in that specific area.
For example, suppose the professor has observed that a student excels in logical reasoning, but struggles with public speaking and analyzing real-world situations. If the student were applying for a program in mathematics, in which logical reasoning is important, they'd likely get a strong recommendation. If the student were applying to a business program, in which it is important to analyze real-world situations and speak convincingly about them, they'd likely get a weak recommendation, or the professor might decline to write one at all.