I don't think it is an issue of the size of subtitles at all, as has been pointed out foreign subtitles are often included.
I don't think it is discriminatory however, in flight entertainment is never part of the condition of carriage, just a nice extra. You are paying for the airline to get you from A to B, which they will do regardless of your hearing ability.
You may moan the lack of subtitles, but then should all IFE also have audio descriptions for the blind?
What about if you don't speak English? Are you be discriminated against because there are no movies in your native language?