But that's the thing. To me, none of the above is "common sense" even in cases where I would personally reach the same conclusion as you when it comes to my own behaviour. F pyjamas are completely reuseable and so are the pens, why would it be common sense that you can keep those plane pyjamas that could perfectly be washed and offered to the next guests and not the blankets which can perfectly be washed and offered to the next guest are even typically cheaper to produce? Why is it is common sense that it is a complete no-no to keep hotel pyjamas when you travel to Japan but not on JL F? Or again, as I mentioned earlier, why is it common sense that you can keep your pyjamas on JL F but not your travel cardigan on JL J?

As for bathrobes in hotels, I fully agree with you that it is a "no" but not because of common sense, rather because hotels invariably leave little notes saying things along the lines of "please, enjoy this bathrobe during your stay. If you wish to take it home, please visit our souvenir shop where they are on sale" or some similar wording. Similarly, why is it common sense that one cannot keep the little plastic ramequins that airlines use on many Y meals (or again for CE desserts) and which BA pays less than £0.10 for, but can keep the earphones that they hand out in WT and could very easily clean and use again (indeed, some airlines do it).

Ultimately, common sense is precisely judged by the fact that an immense majority of people would make the same guess as to what is or is not acceptable. In this particular case, the sheer number of people on this thread reporting to keeping airline crockery or salt and pepper shakers suggests that it is perhaps not quite as "commonly" sensed as you suggest...