The point of most legislation is that people can't always protect themselves. Whether it's from other people, or from large organisations with great power, such as international corporations or the government. Strict privacy laws in more leftist places outside the US protect people from this sort of thing.
The fact is, people don't actually know with whom they're sharing the personal information. They think it's just their friends who can see their status updates / employer / friend network / political affiliation / sexuality. If the CIA called them up and asked them completely straightforwardly to volunteer that information for a national database I'm sure most would immediately refuse outright.
That's not even the main impact. Think about all of the people accessing Wikipedia every day - students, teachers, randoms - who don't read or watch the news. Ever.
All of these people who will probably never hear about SOPA, even if it's passed, will suddenly realise that their First Amendment rights are under threat and will hopefully be incredibly, incredibly angry.
This is amazing in terms of ease-of-use. However, the privacy implications are a bit worrying...
I don't really want to rubbish this, I hate the naysayers when something disruptive comes around (note that Japan rolled out pay-by-cellphone on their subway systems something like 10 years ago) but...
"She saw my name and photo on her iPad, tapped it, and I was done. A receipt was sent to my phone."
So all employees at stores using Card Case can see the name, identity and photo of every single person using card case within a 100 foot radius of the store. Completely regardless of whether they want to buy something or enter the store. What if you don't want random stores to know when you walk past? What if your abusive ex-husband is working at a store and sees your name come up on the list.. etc.
Additionally, if you use the app, Square knows exactly where you are at all times. I'm not at all comfortable with that. Apple had a massive scandal when it was revealed that it's technically possible to retrieve location history from iOS. This is legit human tracking - police and three letter agencies everywhere are probably throwing parties right now.
HN was up in arms when retail giant Westfield was tracking number plates entering their car park. Just number plates.