So.
Is this not deceit ?
It is Indeed, But that couldn't be hidden for a long time!
The husband will know after while and the results will not be as good as she expected (she : the woman who buy the +ve P tests)

Should these types of ads be allowed?
No, But Internet is a big world, You can whatever you are looking for

More to the point, why would you purchase such an item ?

In the case of positive pregnancy tests, maybe a woman will buy this if she was desperately trying to save the relationship if the man wanted kids, and she thinks she will get commitment from him.