What will you define as "valid"?
Using logic I can only conclude that glue is not used or is not effective for water proofing OR there are as you called "other business decision" involved here.
Your alternative means that glue works but Apple uses the water detection stickers to refuse warranty in bad faith, the battery exploded because it was a bad batch => the water sticker is red, sorry no warranty for you!
Anyway the phones are not in reality water proof,so this excuse should not be used by fanbous, let Apple lawyers use it, they are actually paid for it and might even know the reason it is used.