I don't know about other people, but when I think about a shopping list, I don't necessarily think about this requiring a specialist third-party app. My shopping list may just be a text file that I use Notes to open, and it might not be so much a traditional shopping list as a list of presents ideas to look out for, or an email from a friend, or a price comparison website, or a free-text field added to a contact or To-Do or Calendar item (if this is possible) etc. For ease of flexibility, I would rather not have to copy and paste this text into a rotateable app.

So the point I'm trying to make is that one use case, at least for me, is "ability to look at (and manipulate, e.g. scroll) Notes, Email, Browser, Contacts, To-Dos, Calendar applications easily while held in one hand, for example while shopping" rather than the more specific "Have a third-party shopping list app that works in portrait mode".