I don't use twitter, but it always seemed to me that a blue check mark was an "upvote" from twitter, rather than authentication. Isn't this how it works? I don't think I'm able to get one if I asked for it.
Giving General Motors a blue check mark is a no brainer because they are likely to be spending ad dollars. Giving celebrities of almost all levels, including short-lived ones, a blue check mark is also helpful because it has an impact on impersonation which is an attack vector on social networks.
The problem is that there are likely others in this celebrity bucket who are attack vectors, but bring reputational problems. Some pornstar might fall into this category. Twitter are much less likely to want to give them a blue check mark than a less well known comic. Someone can impersonate and attack a larger segment and Twitter's security team would probably advocate for reducing the risk, but the corporate arm of the organization won't go for this.
There isn't really anything nefarious here. It's just a case of the demand being so high and Twitter working out what they want to endorse. If they can get past some of the awkwardness of improving the posture around celebs like pornstars then I'd expect they'd extend the blue check mark to others who struggle. There are a lot of parties left out in the cold here. Things like this tend to be slow to evolve in corporate America.