[ZiiS]

If the manufactures wanted to they could easily make a sealed phone and a sealed battery and just clip them together. The extra thickness of two layers of waterproofing between them is arguable on a fashion phone but irrelevant on a product like this.

[Izkata]

My last flip phone had a replaceable battery and was waterproof: the cover had a rubber gasket and you had to use a coin or key to pry it open.