NASCAR was willing to approve Jones at Bristol based on experience:

Erik Jones had never turned a lap in the Sprint Cup Series when he became a last-minute replacement driver for Denny Hamlin at Bristol last weekend. Jones was flown in from North Carolina during a lengthy rain delay in Sunday’s race when because Hamlin had neck spasms. The 18-year-old arrived 5 minutes before the race resumed, and jumped into Hamlin’s car for the remainder of the event. NASCAR had to approve Jones as the replacement driver, something that isn’t done until the competitor has at least turned some laps. But in the case of Jones, NASCAR was satisfied he was capable of driving. Sprint Cup Series director Richard Buck said Friday that Jones was approved “based upon his recent experience at the track, and we had just gotten a fresh look at him the day before at Bristol.”(Associated Press)(4-25-2015)