There's no specific law that states they have to disclose every single technical aspect of a product -- it's just common practice in computers and phones since people like to compare and play spec wars.
In Apple's case, listing the RAM, number of cores, core frequency, etc, would make it appear that the device is inferior on paper, even though you're comparing Apples to Oranges due to the software and hardware combination. Apps on iOS run much closer to the metal than apps on Android, so they need less resources to get the same job done. Short of "Safari Tab refreshing" that people toss out as a reason to get more RAM, I've yet to run upon a situation where I couldn't get a task done on my 5S with its 1GB of RAM. Developers will develop for the resources they know they'll have.
Apple's spec sheets are interesting at any rate -- for example, they do go into more detail about their cameras in their spec sheets than most other manufacturers. Why? They lead there in some of the technologies used. Any time Apple has a lead or a notable, positive differentiating feature, it's on the spec sheets. RAM, number of cores, core frequency, etc isn't leading by way of plain numbers, so they're not listed.
Should the iPhone 6/Plus have 2GB of RAM? Probably, we're not talking about a lot of money or additional power draw, but it's also not a reason to not buy. As I said above, developers will develop for what resources they have. The phone is no less useful unless you spend all of your time in Safari with several tabs open that you need to switch between constantly, where it's been shown extra memory helps.