Why should they inconvenience themselves, when they can inconvenience WB and have them spend their own coin to digitally remove Cavill's mustache from one of WB's presumed biggest films for this year?
The usual answer to that would be that since Paramount is in an inferior position in the industry right now it might need some similar favor from WB in the near future rather more than WB needed this favor right now.
It's unusual for the guy on the lower rung to reach up and kick the guy above him in the ass.