Unfortunately, a lot of the rooted by default tablets tend to be the cheaper unbranded tablets and there's not a lot anyone can do about it apart from not buy them. It's an awkward area, in that it's not something that is widely known, nor something that most people would even think about unless they are already in the know. All too often the customer does not realise until it's too late.
As you've found out, even "unrooting" doesn't always resolve the issue, so I can only surmise that the unrooting process itself leaves some trace of rooting that apps can still pick up on.