If you put bacon grease on your hands, and had a dog lick it off, would that be wrong? I believe your average dog would gladly do so, voluntarily, and with enthusiasm. And I doubt the dog would care whether you put the bacon grease on your hands or on your feet, nor would the dog care if you found it sexually arousing or not.
We're not talking about holding down and fucking a helpless bunny up the ass here. I believe that the hypothetical dog in this situation is consenting to participate.
If I'm not mistaken, "sexual activity" as defined by bestiality laws, has to involve penetration and/or genital contact. I seriously doubt a bestiality charge would stick in this situation.
To me, this is an example of people who have a tickling fetish assuming that everyone else is going to perceive tickling as sexual, too. But my guess is if you ask a dozen people off the street what they'd think of getting a dog to lick your feet, they'd think it was odd, and probably gross, but not an animal rights violation.