I agree it becomes really silly beyond one or maybe two steps. IBM in the 1930s was about the limit of "second degree hate"; I'd still have been happy to mow the lawn of IBM employees back then. I probably would have had problems doing business with IBM directly if I'd known all the details. The only transactions I'd have done with nazis were sell side death transactions, i.e. real life Castle Wolfenstein.

The Koch brothers probably aren't going to change their behavior no matter how much boycotts cut into their bottom line. If you cut off all revenue to every business they have an interest in, and everyone in the world refused to send them another dime even by buying up their failing businesses at fire sale prices, they would still be richer in their cash holdings than most people ever dream of being. They'd continue to have more influence on world affairs than most small nations.

On the other hand, most large corporations are more interested in money than politics, and politics is just a side game. So a moderately successful boycott of a single or small number of products sends enough of a signal back to corporate headquarters to change their behavior. If Whirlygig sales drop 10% because of their policy on Foozles, they aren't going to wait for Fizzbuzz sales to follow, they're going to reverse course immediately.