There isn't any good reason why an employee shouldn't be allowed to wash his feet. It poses no conceivable health risk to others.
That said, it should be the right of the business owner to determine what practices are/aren't allowed on the premises, even to the exclusion of "protected" races and religions.
Athletes foot fungus.
Now you know.
I'm pretty sure we can all live with the possibility that
i) a Muslim employee has athlete's foot and elects to bathe his feet in the sink,
ii) the employee transfers a meaningful quantity of the fungus onto a sink surface while washing,
iii) the fungus isn't subsequently washed away or killed by soap or other sterilization,
iv) the fungus survives until person X touches the surface and transfers a meaningful quantity of the fungus onto his/her hands,
v) the fungus remains on X's hands despite hand washing/wiping,
vi) the fungus remains on X's hands until he/she at some point takes off his/her socks and shoes,
vii) X transfers the remarkably resistant fungus onto his/her feet, and
vii) the fungus survives long enough to grow and spread.
The 3.5 people per decade for whom the above series of events occurs will just have to suck it up and buy themselves an $8.00 tube of athlete's foot cream.