Your analogy doesn't work. In some cases, it is illegal to modify something you own. Going with the weapon theme, a sawed-off shotgun comes to mind. Even if you have a legitimate reason to make the modification, it's still illegal, in the US, to reduce the length of a shotgun to less than 26" overall and an 18" barrel. Doesn't matter if such a modification could make the weapon more useful during legal use.
Wait a minute - you're comparing wanting to use product features that were advertised by the manufacturer and then taken away, to something that is specifically prohibited by Federal law?
Shotguns are not advertised as having the feature of being able to saw off the barrels to a shorter length. Many people do saw off the barrels to the legal length, but no shotgun manufacturer advertises this as a selling point, regardless of how useful it might be.
Sony advertised that the PS3 product could both run "Other Operating Systems" such as linux, and it could also use the PlayStation Network. Those are both useful features, and they are not violations of Federal law (which your shotgun example would be).
They then updated the software on the product (PS3) such that you could either choose to retain the Other OS functionality, or the PSN functionality, but not both. That is stealing, or if it's not, it's at least the intentional introduction of a defect into the product. Customers should either retain all the advertised functionality of the product, or be compensated for the loss of that functionality.
Here's a car analogy:
You buy a new Toyota Boringmobile. It gets cold where you live, so you buy it based on Toyota advertising that it has heated seats. They also advertise that is has the ability to safely transport you and your family from place to place. Those are two advertised features: 1. Safe transportation, 2. heated seats.
You pay money for the car. Toyota gives you title to, and possession of, the car. You drive it home. You are happy.
Toyota sends you a notice: "Bring your Boringmobile into any Toyota dealership for a free service to make sure it continues to fulfill it's promise of safe (if rather dull) transportation". There's a recall on the tires or something like that.
You visit your Toyota dealership, and they replace the tires with new ones which work exactly like the old ones, but you needed to do that for safety's sake - Toyota's notice to you more or less said so. At the same time, Toyota disables the heated seats.
Wait a minute! You paid for heated seats! But they don't work any more. Toyota says "Well, you agreed to that in the terms of service - it was on page 38 of the agreement you agreed to by driving to the dealership"
But wait a minute, contract law doesn't work like that - they can't take features back without compensating you (Generally in a contract, "consideration" i.e. money, has to change hands in exchange for taking or providing goods and/or services). You take Toyota to court (most likely as part of a class action), and get either money or your heated seat functionality back.
What has happened here is that Sony has stolen functionality from the owners of a physical product that was bought and paid for.
The proper shotgun analogy is that you had a double-barreled shotgun and you could shoot both barrels, or just use them to store two shotgun rounds if you chose to never fire the shotgun. After an update, your Sony shotgun will now only fire the first barrel. The second barrel is now just for storing a spare round. Don't like that your gun doesn't work as advertised any more? Sorry, it had to be done so that you could continue to use Sony ammunition. Except that it didn't, did it?