Why are some watering cans marked as being for indoor use only???

I keep my plastic watering can indoors on the stairs … so I don’t have to bend right down to the floor to pick it up! It has lasted about 20 yrs so far. I have had my money’s worth out of it.

 

I have a really old plastic one that has been kept outside most of its life, no kidding it goes back to the early 80s.
I had to mend it last year, so took pity on it this winter and its spent it in the shed.