Let me put it an other "another way".
Let's suppose Fire_Wraith really thinks Jeb Bush sucks a donkey. So he wants to let everyone know that a vote for Jeb is one step toward the end of civilization. Under the constitution and federal election laws, Fire_Wraith can go out to the town square and scream at the top of his lungs on the topic all he wants. He can even publish ads in the newspaper and on TV. Or put up a web page. As much of his time and treasure as he'd like to spend on the issue, he can spend.
In this way it is just like any other issue Fire_Wraith cares about. He could be advocating for parental rights for Transgender divorcees, or anything else his heart desires. The FEC only takes note when he's dealing with a federal election in some way - and he can't coordinate his activities with those campaigns without calling it a contribution and being regulated.
Now, here's the other shoe to drop: What if I think Fire_Wraith is the smartest guy ever and I'm totally on his side. I want to join in his efforts. So I offer to pay for half of his advertizing and help write the ad copy. I'm doing the same thing that he was doing before. So is he. Only now we are doing it together.
Pre Citizen's United we suddenly were running afoul of federal law. Just because we were pooling our resources for common cause.
Even today Fire_Wraith and I would run into difficulties because of campaign laws. It happens all the time, all around the country and it has nothing to do with corporate oligarchies. Let's say instead of Bush, Fire_Wraith and I were worried about something happening in our town and we got together with a bunch of people from the neighborhood to do something about it. We put our heads and wallets together and get a bunch of yard signs printed up. Ooops. We just violated campaign finance laws. We are now a PAC and have to get ourselves all legal and regulated and stuff. So even though we were only able to scrape together $382.78 for the yard signs (and coffee and donuts for the meeting), we are required to file complicated paperwork and collect information for the regulators on all of our contributors and all of our expenditures. Even though this would cost way more than we are spending on our actual political activities, and even though we could barely muster the energy to get the yard signs deployed.
This is what campaign finance laws look like in the real world. Often a group like ours would skate under the radar and not be bothered by regulators. Unless, of course, somebody got their undies in a bunch and decided to do something about it. The example above is based on a real story - I think it was Colorado a few years back. Some neighborhood group was opposed to something the city was doing and tried to oppose it by pooling their resources. The powers on the other side used the levers of government to silence them - audits and subpoenas and prosecutors all drowned the group, costing them many, many times what they were planning to spend on stopping their government.
This can reach extremes when your political opponents happen to work for government as prosecutors or regulators. In this case a prosecutor used his power to silence (and bankrupt) someone who was raising money for the opposing party. Someone who ultimately was found to be operating perfectly legally, but was harassed and silenced for 5 years, using campaign finance laws.
There's a lot to worry about here, not just on the "corporations are evil" front.