You probably wouldn't use multiple tables or multiple columns either and just have a single table that stores document-like rows... which vaguely reminds me of something.

PostgreSQL is a pretty good document store, and when you need it you have the relational model available and integrated.

Foreign key constraints are not business logic, they're part of a sound and sane database architecture (just like basic indices on heavily-queried columns). For an overwhelming majority of use cases, tables should have them; one needs a very good reason why a table shouldn't have them.

Or arguably data types.