Any electronic device has a temperature threshold in which it is safe to operate and should be shutdown if the temperature goes under or over the limits.
If the iPad doesn't shut down in this situation, it could do permanent damage to it.
Please remember that this is not an iPad specific problem, every electronic device (that does not have a cooling fan) should be used under the sun with care.
I once forgot a VHS cassette in my car on a sunny day (around 80F), and it actualy melted and became unreadable. The sun might not look as it is heating the device a lot, but without a fan the heat can become unbearable.