Does she also not consider that the two of them going out for a shopping trip could be a massive security breech?
She might not have known at the time. Because she was very new to at that back then. For the VIP to go out alone, the security detail would have had to pre-approve the trip as low risk - what the proposed outing is like, and is it genuinely random enough that no-one could predict where they'd be. Yes, phone cameras abound, but if people have snapped her (by chance) the pix aren't making their way into the public domain. And most people would think they'd seen someone very like her, not the actual her, especially if she grunged down a bit (not embarrasingkymso, but hair less glossy, understated make up, leave off the rock of an engagement ring, choose clothes that blend in)
The two of them isn't the same trip, and would need a new security OK
OTOH, if security was with her, they'd still,want to re-plan and perhaps add a person as there would be two principles to look after. It might not take long to re-arrange but is still taking up time. And they don't just go wandering off - security will know where they are going and how long they expect to be out. It's nothing at all like an ordinary person popping to the shops and browsing wherever takes their fancy
Perhaps those are the sort of restrictions in Royal life that it appears she found so difficult.