I can answer my own question: Apple doesn't want people to share these devices: fewer sales.

Not saying they won't look at it in the future but We have a family iPad and its no problem. My wife can see my email but she's my wife so I honestly don't care. Our neighbours have two between six of them and similarly have no issues.

People with a tech background worry about this more than most people but they also tend to be the sort of people who have their own devices. For normal families the current situation is a relatively minor problem - considerably less of an issue than both kids wanting it at once I suggest.

I also tend to enable Airplane mode by default when I hand my phone or iPad to my daughters.

Apple has trapped itself into a corner with the Apple ID. Apple puts on regular seminars for local school districts every 6 months or so in my area about how to integrate iPads for K12, and it is just brutal. At least 60% of the answers from the Apple rep is usually "you can't use it like that".

The hardships all come from the way that the Apple ID works. Apple wants the iPad to be a "personal device" for one person, but obviously in a school setting it doesn't work that way. The workarounds they have for school districts are laughable to the point that the Apple rep(s) always admit it to us when talking about manageability, security and tracking.

I love my iPad, but its just not enterprise/public school/multiuser friendly yet.

Bingo.

Apple wants people to share iPads only to the extent that doing so gives other people a trial of the product, and ideally, a burning desire for it. Apple doesn't want the kids sharing Mom's iPad; Apple wants kids bugging Mom half to death for their own iPads.

A and B do not contradict. B does not invalidate A.