I'm sure the leagues are considering multiple scenarios.
Blues season ticket holders, for example, have been told their credits will roll over to next season if games don't resume this season.
MLB is, at least for now, standing pat until it figures out what this season is going to look like.
The big difference is postponed/cancelled.
Postponed means there's a chance it could still happen.
Canceled means it's off the books, and money goes back.
The London Series was canceled, so those have been refunded.
I know because I had family members who got tickets, and they have received refunds.
The leagues deserved some grace period as they reacted to a totally unpredictable scenario, but they will face more bad PR the longer they hold fans' money without answers on what seasons are going to look like, especially as most of the options being discussed involve no-fan scenarios.
Some people are in a pinch and could use that money now, and they need it a lot more than the leagues or third-party ticket sites.