There is much more to this than just "owning one share".
You make it sound like he was some retired grandfather who happened to have a torn and tattered solitary stock certificate stuffed in the attic somewhere.
BTW, Joe Kennedy and other American elites were in bed with fascist regimes like the Nazis and Soviets. This is well-documented in many sources. If you like to read, I'd recommend G. Edward Griffith's "Creature from Jekyll Island". He covers many of those connections.
Prescott served as Artillery Captain in WW1, and his son George H.W. Bush was serving in WW2. This is hardly grounds for the treasonous claim you make.
Think about it, if we were to go to war with China in the next few years, would you be treasonous because you owned 1 share of a stock that was "affiliated" with a Chinese company? Would you be a traitor because you shop at Walmart?
Yeah Prescott should not have owned that one share and served as an honory Director, but the level of blame he has is the same as every person who shops at Walmart and owns stock that deals extensively with U.S. enemies (and there are a lot of them).
Yes i know many of the Rich industrialists were in bed with the Nazis, but Prescott Bush was hardly one of them.