"Fare evasion" is one of those "broken glass" policing problems; if I recall correctly (from a 1990s New Yorker profile), the MTA didn't really manage to begin cleaning up its subways late in the last century until they began going after fare evaders.
The last few times I took the Metro (I am an occasional user as I live and work in Herndon), the sheer amount of fare evasion going on was a surprise, and it was mostly young men (white and black) and statistically those are the ones most likely to be vandals or engage in theft and violence. I don't think homeowners from suburban Virginia or Maryland would want to subsidize the Metro if they didn't feel comfortable riding it; a sense that it is clean, maintained, and safe is key to that and preventing fare evasion is an effective way to provide that sense.