Well, it depends on what you mean by "sponsor". Corporations are still limited in how much they can directly give to a candidate, so they cant technically fund a campaign.

What they can do though is "independently" voice their "opinion" on who should win. So, they cant put money directly into anyones pocket, but they can now spend as much money as they like promoting the candidate, which in effect nullifies the rule about direct contributions.

The reason they did this is because, in US law a corporation is considered a "person" because it "represents a group of people." and since every person in the US has basic given rights, the logic is that these "people"/ corporations should be given those same right, namely free speech.

So its like they are saying "you cant bribe people, unless you use this giant loop hole which lets you bribe people indirectly." and obviously the reason that is a terrible thing is because this will only lead to further possibilities for corruption in politics.