Dear Thakonkorn,
Thanks for writing in. I'm happy to straighten this out for you.
It's clearly stated on the website that Partner UberBlack will have to switch to UberX when required. Please refer to http://www.uberbangkok.org/uberblack.html for more information. Also, Uber Bangkok team always makes sure to provide such information during the application process at Asoke office.
Please be reassured, Thakonkorn, that Uber is unique service. We are evolving the way the world moves. By seamlessly connecting riders to drivers through our apps, we make cities, like Bangkok, more accessible, opening up more possibilities for riders and more business for drivers.
Uber has come up with this method to provide opportunities for Partners to complete more trips. That said, enabling this view for all UberBLACK partners can help to increase income for those partners while also adding additional supply to uberX in Bangkok.
I hope I have made this clear for you.
Have a great day!