>negligible bandwidth requirement to buy vote on their spam.

2 things here. 

In the order of occurrence, they are mostly

1. SPAM / Plagiarism
2. Buying Vote


While vote buying on bad content is definitely a bad thing to do, the root of the issue is still item number 1. In the current form, its not too difficult to tackle too. So why issue 2 is addressed first than the issue 1 which will completely eliminate issue 2 ?