Because the teacher says the definition for direct substitutution property can only be used for polynomial and rational functions.
Theorem: (Direct Substitution Property) Suppose f(x) is a polyno-
mial or a rational function and a is in the domain of f.
http://web.viu.ca/wattsv/math121/Ove...bstitution.pdf
He said that "using the direct substitutution property for trig functions won't be theromatically (if thats a word) correct."
Seriously someone needs to revise the direct substitutution property so that it will be "theromatically" correct for trig functions.