The thinking behind that was that passengers boarding at Watford travelling towards St Albans had either come through the barriers at Watford, or Euston, Hemel, Bushey etc and therefore already had tickets, Very few travelled from say How Wood to Park Street.
Coming the other way of course there were no ticket offices at St Albans or any of the stations, so it was a "pay train" and commission-wise it was more lucrative to sell tickets from St Albans/Park Street/How Wood etc to London than it was to get a child single from Watford North to Garston!