Coupon Etiquette

Is it against some written or unwritten rule to take/use discarded coupons? For example, if someone leaves them behind at the store?

I often leave coupons that I will not use for someone else or give them to someone I know. I don't accept money or anything for this. Most coupons are manufacturer--not store coupons--and require a purchase anyway.

Please clue me in. Was made to feel like a thief for glancing at discarded coupons to see if they were anything I might buy. Decided to leave them behind.

Looking for some insider couponing advice. I always have problems with this store though.