Just to add some context, succulents are ridiculously easy to propagate from a fallen leaf. You literally just put them somewhere until they sprout roots and stick that in a pot. Takes a week, two max. That said, succulents also take years to mature. Your little sprouted leaf is going to stay small for about 2 years and by 4 or 5 it'll look decent. So it may be "theft", but by the time you've gotten a plant that's on par with the one you got a leaf from, years will have passed and you'll have likely spent money on pots, soil, and fertilizer from the same store you "stole" from. It's like pinching a small bottle of paint from a craft store then turning around and spending $100 on canvasses and brushes

If you weren’t going to buy it anyway

That can get into rationalization territory.