When you let money play, money wins.

A direct money transfer is an unmediated transaction of money and upvotes.

Normally the transaction is mediated by content, as follows:
User/creator effort + money investment = content, which => upvotes

Unmediated:
User effort + money => upvotes

This is caused by the fact that the overall quality and volume of content here on Steemit is low enough that it allows small players to disrupt proper curation and quality-filtering by purchasing upvotes.
This would never happen on Youtube for example because any reasonable amount of money spent on buying views would not put you at the top of the trending charts (because competition and overall quality & volume of content is too high).

tl;dr
If steemIT grows enough, buying upvotes will go away by itself.