I instantly (and incorrectly) assumed that this was some conspiracy theory-laden story about how big pharma doesn't want to stop cocaine use and the war on drugs is too profitable and etc. etc. etc., but thankfully I was wrong. Here's the key line from the story:

  the dilemma is clear: What would happen if the media got
  wind that inoculated drug users were buying even more coke
  than before. It would be a public relations nightmare for
  any business.

So that's it - and it makes sense. Vaccine makes cocaine less effective, so people buy more (in the short term) to try and get the same high, until they finally realize that doesn't work and give up using it. Long term good, short term PR nightmare.