Some hosting companies (such as Vox) host thousands and thousands of websites. It's almost impossible for hosting providers to keep track of every website they are hosting and who they are owned by. Banks deal with financial transactions in the millions, or billions. The ramifications - both legal and financial - are far far greater for them. A R99 hosting package is not quite comparable.
Also, the banks hold far more clout, more than any other industry. Pulling the plug on Gupta businesses has real ramifications for the family, their businesses and JZ783. Would be lovely if absolutely every business cut ties with the Guptas, but in this case, I wouldn't hold it against Vox.