"Discrimination" is a term with a lot of meanings, not limited to racial discrimination, sex discrimination, class discrimination, and the other major categories you're referring to, one of which I suspect is discrimination against the disabled. There are many people who
cannot drive, whether because of a mobility impairment, a vision impairment, or a neurological impairment (such as epilepsy). And the way our society's infrastructure has developed, and the way it has dismissed many alternative forms of transportation, marginalizes and harms those people. That's discrimination. That's what the ADA was enacted to combat, though the ADA only addresses some of the problems with transportation that such people face.
I suspect you also include religious discrimination among your standard definitions. There are also people who
cannot drive for religious reasons. The Amish, for instance, and strictly observant Muslim women. Their right to practice and observe is protected Constitutionally, and while there is no Constitutional right to alternative transportation it can easily be argued that they are marginalized by society's disregard for their needs. That is discrimination -- it's not necessarily the same legally-defined discrimination, but the word's definition is hardly restricted to its legal usages.
And yes, there are also many people who
prefer not to drive. Some support public transportation for its economic benefits, others for its environmental benefits, and still others for how comprehensive public transportation reduces discrimination against the above classes of people. You may feel that it is inappropriate to consider these people "discriminated" against, and here I agree it is not a word usage I would have chosen myself, as I feel the word is most strongly associated with the long, difficult struggles faced by many groups in recent American history -- it's a very "loaded" word. But I don't agree that it is an inappropriate word, as far as its "dictionary definition". It has an extremely broad dictionary definition (here I cite Merriam-Webster). I think GML was well within this definition in what he said.
Main Entry: dis·crim·i·na·tionPronunciation: \dis-ˌkri-mə-ˈnā-shən\
Function: noun
Date: 1648
1 a : the act of discriminating b : the process by which two stimuli differing in some aspect are responded to differently
2 : the quality or power of finely distinguishing
3 a : the act, practice, or an instance of discriminating categorically rather than individually b : prejudiced or prejudicial outlook, action, or treatment <racial discrimination>