If a developer argues that these constraints are not necessary because the code manages the integrity, it can be argued that after implementing these constraints in the database there will never be a violation of the database constraint. So what's the problem with having them in the database. Furthermore, can it be guaranteed that all access to the database will be through the application? Perhaps. It can be guaranteed that all access to the database will be through the database.

On the topic of purchasing products with their proprietary databases, perhaps if enough data architects were involved in the purchase of these products and if enough of us said no to those which do not adhere to these constraints, the software vendors would have to build them in or not sell their products. Those are a couple of pretty hefty ifs, but worth pursuing.