@Terisonen
I'm not sure why you think that fairness to other cultures should limit the ability of the Rohirrim to ride and fight from horseback. Mounted Warfare is what defines the Rohirrim and makes them different from the, otherwise similar, Bardings. I have created two, slightly different, mounted cultures, The Leofrings(from TOR supplements- see the Houserules forum) and the Marhathiuda(see Other Minds issue 14).
There are several things I considered when creating them.
1) There are some cultures where horses are rare and those there are, are usually workhorses.
2) There are some cultures where there is an equestrian class, capable of limited mounted combat.
3) There are some cultures where mounted combat is the preferred method of warfare.
4) There are the Riders of Rohan, who are probably similar, but slightly better than those at 3.
I created several Traits for the first group to facilitate non combat riding, several Mounted Combat Masteries for the second group and a whole bunch of Cultural Blessings, Rewards and Virtues for the third. This represented a scaled increase in horsemanship and mounted combat ability. I suppose that my take on it is that if you are not a trained rider, you have no business being on horseback when the sword swinging starts. Leave cavalry actions to those who are trained in it, or born to it.
I will be very surprised if the official Rider Culture does not have a Mounted Cultural Blessing as well as several Rewards and Virtues.