It's the manner with which they're conducting the tests. Do you think nuclear testing should be done in private by an authoritarian regime, or do you think it should be done with the acceptance and supervision of the world's scientists?
US intelligence was completely wrongfooted at the time of India's five Pokhran-II tests.
Firstly, no self respecting nation is ever going to wait for 'acceptance' and 'supervision' of some other nation. In fact that very thing implies some sort of master-slave relationship. Countries act in their own interests(just like every other country).