It violates the Google style guide[0] to use unsigned integers. Plus, it'd only defer the problem for another ~2 billion views or so. May as well switch to signed 64-bit integers if you have to swap out the data type.
And I wasn't suggesting using unsigned 32-bit integers now, I was curious why they weren't used before (the number of views seems to be a nonnegative quantity...).
[0] https://code.google.com/p/google-styleguide/