It made sense when the rules were "below ground - smoking banned",
"above ground - smoking allowed".

When LU brought in a total ban, how could be "dangerous" to smoke at
East Ham and Upney, but not at Barking? In fact the above ground LU
smoking ban is widely ignored and never enforced (frequent PA
announcements are useless as smokers just carry on regardless).

Until recently they were still bombarding LU passengers with repeated
messages saying "it is illegal to smoke in any London Regional
Transport bus station" which patently wasn't the case (see Finsbury
Park, Becontree Heath, Victoria etc), and just what is "LRT" anyway?