I havent researched this question in particular, but I used to teach a course (at a prominent university) called Jazz and Literature. Of the many issues that were discussed, of course the issue of race came up all the time. While I am often uncomfortable getting into racial stereotypes and distinctions, Ive always theorized that the name of the tune depends on the arranger or singer–i.e., Billie Holidays version (my favorite) includes the S in I LoveS You Porgy, while other versions do not. In sum, (and perhaps a linguist would know more about this) I find that some African-Americans, just as a simple matter of their speech patterns and use of the language, will often add an S to verbs, and especially the verb love (eg., I sure loves me some gin–which you can hear in several blues songs, etc.). However, this tendency certainly cannot be generalized and therefore does not apply to ALL African-Americans, obviously; further, I think this tendency has probably faded quite a bit from earlier years in which it seems to appear more commonly.
In my view, this has nothing to do with proper or improper English, but, rather, has everything to do with cultural differences and the complexity of language. Just food for thought. I should research this more, as Ive always wondered about this subtle difference in the titles of such a lovely tune.