If advertisers have no control over which content their ads appear near then they can legitimately claim, when criticised, that they are unable to pick and choose.
By giving advertisers control, when criticised, the critics can use that same control in order to pressure advertisers to limit ads to inoffensive content only (which, frankly, is almost nothing anymore).
But with the direction "being offended" has taken (i.e. someone on twitter will take offense to anything, and they feel they have a legitimate right to never feel offended) it won't be long before most content is inaccessible.