The easiest answer to the question - no one has sued 2K yet about tattoos. EA lost a lawsuit about tattoos (NFL Street 2 featured Ricky Williams and his tattoos, the artist of Williams' tattoos sued and won), thus the company's different approach being one of avoiding litigation.
It's also possible that WWE requires wrestlers to get rights to all their tattoos before appearing in any print / digital media whatsoever, given how prominent tattoos are in wrestling in general. This would eliminate the problem altogether before it even gets to 2K.
Finally, there have been instances of tattoos being removed from WWE games. CM Punk has a tattoo which strongly resembles a Pepsi logo on his left shoulder, that tattoo never appeared in a WWE video game.