They cannot prove the existence of the unicorns, and thus they cannot define the unicorn's reaction to the shoes being worn. Perhaps the unicorns are fake, perhaps the unicorns do not mind sharing the shoe space with the feet (and are capable of compressing), or maybe the unicorns even prefer to be compressed by feet.

We don't know, so we shouldn't make it a universal moral rule, much less a law.

EDIT: Besides, it's not like religious people have never committed acts antithetical to their own religion.