Corporation != large corporation. A small neighbourhood family shop is a corporation if they've legally separated their personal and business finances, which is usually advisable. Limiting individual liability and being able to share ownership were the original reasons for allowing corporations. This is true for small and large corporations.
Corporations also exist for the good of society. They allow individuals to limit their liability and share a risk with others. Patents are granted to individuals, but those individuals are free to bequeath them to the corporations they are part of. Or they have freely signed an agreement that they will bequeath them to the corporation.
Now I agree the original reason for patents wasn't to encourage corporations to do R&D. I only gave a nutshell outline of the argument: I wasn't trying to be historically, philosophically or legalistically thorough, so I don't think this kind of criticism is very appropriate. I'll happily admit being wrong, but it is certainly not 'completely wrong': corporations were never restricted from being granted patents, even though that has been suggested. The common argument is the same argument given for individuals: protect the inventor or his corporation.
Neither corporations nor patents came into being with immoral intentions. Both ideas have actually proven quite successful. Unfortunately, we are now seeing some excesses of (the combination of) them that suggest the rules need to be changed.
