Default enablement for custom products changed in Katello 4.9 to make the Simple Content Access experience easier. If you want to get all of the context, see [RFC] Making things easier when working with custom products & Simple Content Access (SCA)
Long story short, there are plenty of situations where you want to restrict hosts’ access to content, and there are plenty of situations where you want to allow it. And some where you want to allow some content, and disallow others, and have it all be in the same content view. I’m sure some other users on this forum can give examples. With content overrides, this is all quite customizable, and it was determined that disabled is the best default for custom products in the context of SCA.