This.
They're doing extra work to provide extra content in an already massive game. It takes a LOT of time and money to create just one character alone.
Charging an extra couple of quid isn't too much to ask. Back in the day you had to rebuy the whole game to get an updated version. These days you get free balance patches and new characters can be bought individually when they are made.
No-one is complaining about baiken being added to GGXrd even though she's a mainstay character, because they've already accepted and understood the "we'll add more characters but it costs us money so you need to pay us" idea.
Well...this, and ASW is finally allowing us to upgrade an existing version at a reasonable price rather than buying an all new version for 50-60 bucks.