It is the first time I such rule in a sport betting website. Major bookmakers ,especially in the UK which is considered as the country with a lot of small bookies, bet365, betfair, betathome ,bwin ,eurobet, and many others , if the game do not start or is interrupted during gameplay, the bet usually is a VOID.
Anyway it looks like this is not Nitrogensports fault, they have stated it in the rules. I rarely play tennis but it is good to know the rules, as honestly I never read them there, I took them for granted to be the same as 99% of other sportsbooks.
These are the rules on retirement in Tennis at a variety of big bookies. It's actually much more split than you make it sound between voiding and settling.
1 Ball Served Bookmakers (player must serve just once for any bets to stand)
BetBright
Bwin
1 Set Completed Bookmakers (1 set must be played policy for bets to stand. If one set has not been completed all bets are void and you get your stake back)
Betdaq
Betfair
Betfred
BetOnline
Betsafe
Betsson
Coral
Ladbrokes
Matchbook
Paddy Power
Pinnacle
Smarkets
Sporting Bet
Unibet
888Sport
Whole Match Completed Bookmakers (whole match must be completed or bets are void)
10bet
188bet
Bet365
Bet-at-home
BetVictor
Boylesports
Sky Bet
Stan James
TitanBet
William Hill
Winner Sports
NitrogenSports would be in with the 1 Ball Served Bookmakers.