Does the country where a credit card is issued affect the content offered in iTunes (with all the restrictions for that country)?
Yes it does.

Think of iTunes Store (and Apple App Store) as having separate instances (stores) for each country that Apple Inc. offers the service for. In every country/region iTunes Store has its own offering, language, regulations, pricing, availability, age restrictions, etc.

Apple recognises users by either method of payment or their billing address and allows usage only of the iTunes Store of the region to which an account belongs.

A given method of payment (credit card or prepaid card) is valid only for the country it is issued for (at least at the moment of registration, it can be changed later).