The law says "in furtherance of a business".
It doesn't matter if you take money, a hot dog, or a hand shake. It is the intent to further your or anyone else business makes it illegal.
Not exactly . . .
Actually the law doesn't mention furthering a business - although that is one of the FAA's various examples of interpretation.
BINGO!
The FAA unknowingly "screwed the pooch" then they tried to dumb-down the actual law. I don't know if they intentionally created the confusion or if those who were tasked with interpreting the law just thought it would "be a HOOT" to confuse us.... either way it left a LOT of room for error.
To make it simple look at it like this... If any portion of your flight isn't for hobby/recreation (and this can only be for you, by you, benefiting your enjoyment) then it's a CIVIL flight. If you go by this simple rule you'll see you could never "fly for someone else" and it still fall under recreational. The moment someone says, "Hey can you fly your UAS for me and. . . . " it's not recreational/hobby.