Pepto Bismol (kaopectate) has an asprin like property to it. this is why its not recommended for use.
Bismuth Subsalicylate (Pepto-Bismol®, Kaopectate®)
it can also have effects on other medications being taken.
it can thin the blood
it may not be doing any good at all, and therefore your just giving your dog something it doesn't need.
asprin is the main concern. it is a blood thinner. If what your dog has is worse then an upset stomach the asprin is going to effect blood results and bleeding time when taken to the vet.