It is not the EPA making the requirement. It is done by state rules.
Wells not in service are not being maintained. They will eventually begin to leak. Many older wells were abandoned when the production no longer paid to keep running. They began leaking oil and other reservoir gas and fluids into the drinking water levels.
Most (all?) states have some sort of Oil Field Cleanup fund, typically paid by permitting/use costs of newer wells.
More info can be found at:
WELL PLUGGING PRIMER
http://www.rrc.state.tx.us/media/6358/plugprimer1.pdf
RAILROAD COMMISSION OF TEXAS
If their was sufficient pressure to bring the oil all they to the top wouldn’t still be in production? Wha?