A little testing shows that this appears to be fairly specific to the "align" environment from amsmath. For example,
works fine. I'm not sure why
align* doesn't work (it doesn't work without the star either); I suspect that the myriad expansions and aliases surrounding the align environment do something a little more complicated than a "traditional" LaTeX environment. However, I have found something that might just achieve the same objective for you:
x^2 &= y^2 \\
z^2 &= t^2
Hope that helps!