"Illegal?" Please cite the chapter and verse of the appropriate civil rights laws that states that solo travelers are a "protected class" with respect to discrimination.
By the way, I am often a solo traveler myself and wouldn't find paying an extra fee for preferred seating to be "discriminatory, offensive and illegal." Of course, if the carrier screws up and I couldn't get my preferred seating, I'd want a 100% cash refund of the extra fee.
If "paying for it" would guarantee the service I would expect, I might also be willing to pay, even though I do understand why it's felt to be "discriminatory." Why should a solo person be required to pay for something a couple or family get for free? "Legally" discriminatory? Maybe not, but discriminatory nonetheless.