Because it reflects on the type of person, especially when you only got a 10 minute interview with them. You cant judge someone on 10 minutes, so you google them to try and find more. For a normal person, you would find their facebook or any other social media. That is where you will find out who the person really is, uncensored and unadulterated. Does the person post things like "Graduation!! So excited!!", with a picture of them in their cap and gown, or "Graduation night was so lit!", with a picture of them shit faced with weed and bottles in the background? There we would be able to see what type of person they really are. True, it may not reflect them as a whole outside the pictures, but that's all recruiters have to go on. That's why everyone ought to be careful with social media.
However with the person above, we won't get that far. We would see their name first thing in the news. And upon further investigation, it is not a person you would want to hire. Most likely since it would severely reflect on the type of person you are, but if working sales, could affect profit as well. That's when we would get "Oh you're that guy that said the stuff about women's rights. Can't believe this place would hire you, I'm not coming back here again". So that's why checking online is used commonly in hiring methods. It reflects the person truly, and shows what type of employee the person may be.