It's easy to say, "don't do x unless you know what you're doing," but many users will think that they know what they are doing. Effectively (it seems to me) the Nokia warning says "don't use your tablet normally; if you do, whatever happens is your fault". It is not unusual to install programs not from the manufacturer; I have done that on every computer I have ever had starting with the Sinclair I got at k-mart. And this is a little computer, not a mere phone.

So, I think that this problem will be solved and users will usually not encounter it.