They key words here are I expect. There is no reasonable expectation for Adobe Encrypted DRM to mean ADEPT encrypted ePub. At best, it is ambiguous and means and ADEPT encrypted document. It is your responsibility to check whether that means ePub or PDF.

The milk analogy doesn't really work out. Even ignoring the likelihood that milk has a legal definition (the maturity of the product, anything to do with health and safety are more highly regulated), consumers pay more for soy milk. The producer wants you know that it's soy milk in order to get the premium price, so they will advertise it as such. This is not the case with ePub vs. PDF. The products are considered equivalent. To people who are reading on computers or tablets, they are equivalent.

A much better comparison would be buying a computer from a consumer electronics store or office supply store, where many details about the product aren't listed. If a consumer doesn't ask about those details, it is the consumer's fault for not doing due dilligence.