Rationale (April, 2013):

9.12.2 [dcl.align] paragraph 6 requires that the alignments be “equivalent,” which in a dependent context is specified by 13.7.7.2 [temp.over.link] paragraph 5. The expressions in this example are not equivalent.

Although 9.12.2 [dcl.align] paragraph 6 requires that all declarations of a given entity must have the same alignment, enforcing that requirement for class templates would require instantiating all declarations of the template, a process not otherwise needed. For example:

   template<int M, int N> struct alignas(M) X;
   template<int M, int N> struct alignas(N) X {};

The same problem would presumably afflict any attribute applied to a class template.