No problem with season ticket holders getting in free - they buy a ticket to see every league game played at home.
The terms and conditions re. abandoned games are up to each individual club, however, Swindon do not have a clear policy included in their fans charter, so make it up as they go along. A tenner reduction for the re-arranged game would have been fine, and I'm sure that they would still have covered their costs as the majority of people would take up the offer.