Well, not really.
Sony require proper infrastructure to support multiplayer available to us. They need to buy hardware to support their servers (and everything else that comes with it, like the added cost of electricity each month), they need staff that can take care of the hardware and they need support that can help people with related questions to the network of Playstation 4. It's literally a chunk of cash piled on top of their other needed payments.
But also because they are a very happy company right now. With how shitty Xbox is doing (not even their recent "pulling-all-policies" will clean up the entire mess), Sony can literally do anything and most will buy their console. What company (their primary goal is to acquire cash, don't forget) wouldn't want an additional cash cow each month if they were able to?
Also, it's just 5 dollars each month (source: ...my cold tea), it's like one sandwich on the street. Come on mate.
Come on.
You're not that cheap.
...Buy me hot tea instead then, you can get some good quality for 5 bucks.