It's not stealing if you plan to do the right thing with it, and of course as long as it's genuinely thrown away by the airline and not reused. I am in the process of actively accruing as many amenity kits as I can, both personally when seated in premium cabins and from like-minded colleagues:
Here is why, and I encourage all of you to get involved too:
https://www.facebook.com/elbh.co
I go as far now as even taking my own soap to hotels, in order to pocket the unused amenities.
Oh, and in terms of the way I feel about Y pax taking PE & J amenities if they want to, my feelings are go for it. It's the same way I feel about letting in so-called 'untitled' people into lounges or to share any other benefits that a lot DYKWIAs just take for granted. I'll share whatever I can and I'll guest as many as I can as much as I can.