People bought houses because they couldn't share them with alts. They still can't. It was allowed at the time and in many cases housing wards sat open with many houses going unclaimed for months. The team recognized that and they weren't going to penalize people who simply followed the rules and did what was permitted at the time.
Changing the rules does not make those people evil and they shouldn't lose what they earned.
Some will continually want to take away from others but the team has made that decision to grandfather and they're not about to change their minds. It doesn't matter how many times some come here and say they should they aren't going to.
Current rules also allow users to purchase housing on other servers as well. One FC and one personal house. If those rules were to change later to one house per account those following the rules now shouldn't have to pay or be demonized because they elected to do what is permitted. Changing rules should and will affect future purchases just like now an alt cannot buy a house if there is one character on that account who owns one on that server.