So by your logic, it is more pragmatic to believe with certainty that no alien life exists anywhere in the universe, than to admit the possibility that it may exist. You think that certainty about things we have no knowledge of is more rational than skepticism and doubt?
It's a probability thing - a probability based roughly on the knowledge extrapolated from other principles and information.
In the case of the space teapot - knowing that teapots are pretty much, necessarily of human construction (as it is a human cultural artifact), and knowing that there is no other sentient life form in our solar system other than ourselves - and assuming aliens have a very low possibility of entering our solar system...
It's only fair to assume that the presence of a mysterious teapot floating between a couple planets in this solar system is extraordinarily low - so low as to be practically impossible.
Similarly; assuming material laws, the unmeasurability of god things, the improbability of guessing the correct god out of an infinite pantheon... theism is also extra-improbable, leaving room for only a-theism (in this case, I mean in the sense of independent of theism, rather than anti-theism).
But aliens on the other hand - assuming the probability of abiogenesis, with the vast perceptibly infinite (as in super large ^2) universe, is not at all improbable. Although you can make the argument that our chance of encountering aliens is very very improbable.
I think... if we qualified our statements with statements of confidence, we'd have a lot fewer arguments on the internet (maybe also a lot less 'fun'?). It's an unfortunate part of human nature that we're more ready to take the easy route of misinterpreting or going for the low hanging fruit - in order to 'win', or 'assert our dominance', rather than providing argumentative charity and interpreting arguments in the strongest form that you can percieve (and muster an argument against).
At worst, you restate your charitable interpretation, and it elucidates and informs both parties in a manner that hadn't previously been considered.