Presumably anybody with a freedom pass (over 60 years old) or a season ticket can enter and exit the station for free, using the footbridge to cross the tracks.
I can imagine it may be difficult for members of staff on one side of the track to let somebody onto one platform, without the possibility that a colleague on the other side will try to prosecute for fair evasion.
The work on the underpass is in the long term interests of all pedestrians and businesses in Forest Hill, but it inevitably causes some disruption while the work continues.