Why is a pregnancy test 'Invalid' if taken apart?

Why do a lot of women say pregnancy tests are invalid if taken apart?

My question is it's just a Plastic casing around a strip test, for ease of using it as a mid-stream urine test. Inside is just a strip.

If one carefully removes the casing and looks at the strip to see faint lines (because the plastic cover many times causes reflection or to take a pic), how does it turn the test result invalid? The line stays exactly the same as it was before, it's just visible slightly better.

What's the hidden logic here??