Private firms contracted to local councils have been notoriously corrupt, but the arms sector is equally notorious for overcharging the government.
2
The primary drawbacks of off-board chargers are that (1) they are less convenient than on-board chargers and (2) they usually need to be personally monitored throughout the charging process to avoid overcharging.
3
Then it's easy to see who is overcharging and who has a seriously competitive price.