Companies like money.
cuz console users let them
Cause gamers were fukking idiots last gen and we're like 'yeah, I'll pay for XBL cause it's way better than PSN' instead of calling Microsoft out on their bullshyt. Now, we're stuck with having to pay for online for every system as all the console companies would be idiots not to charge after the success of XBL
And that's really all there is to it.
Paid online for PS4 is exactly the same as free online for PS3 as far as service goes.