Cedar Point is not discriminating against anyone. I'm sorry that your father found he could not fit onto rides, and it certainly is a touchy subject and a sensitive one, but the fact of the matter is that amusement companies must manufacture rides to fit the majority of riders. By it's nature, this will leave out riders of extreme size, whether tall, short, or heavy. To equip coasters with longer belts and so forth could begin to endanger riders that are not so large, because the restraints would not fit as snuggly as safety must provide.
This gets to the point of where do you draw the line? If you were to equip coasters to handle (let's use height as an example) riders of 6'5", then riders who are 6'7" would be the group complaining. Then if you expanded the restraints to fit them, those few individuals who are even taller would have cause to complain - you did it for them, why not us? And those riders who are shorter would not be able to ride, because the restraints would not fit them properly.
There is no absolutely correct solution to this issue. Amusement companies (and Cedar Point does not build the coasters, so in the most basic sense they are not the ones you ought to be concerned with) need to make a profit, and they do so by targeting the majority with their attractions. ddogg has the right advice: if a person is having such trouble that they cannot fit into any coaster, even the newer ones that are designed more generously, then they ought to be considering their health and well-being, rather than the roller coasters.
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/