Discrimination per se is never illegal, otherwise you couldn't use things like education, experience, or skill level to decide who to hire/promote. It's discrimination against a protected class which is (often/normally, depending on where you live) illegal.

While there are protected classes that benefit from flexible working locations (people with some disabilities, for example), working from home in and of itself isn't a protected class, so there's no real chance for a lawsuit on that grounds.