I'll try and use an analogy so you can understand.

Mastercard give preferential rates to customers who shop at stores that are partners with them. If you use a visa you will not only NOT get that preferential rate, but it will actually cost you more money than it should.

Shell Oil decides to make cars. If you don't put Shell brand gas in that Shell car then you will get horrible mileage due to a design to ensure you only use their brand of gas.

Nvidia includes code that makes games run like crap if you don't use their cards!