I'd guess one of two things. One is a conversation that goes like:

"I want to send letters to everyone who doesn't open our emails."

"We can't really detect that. We could add a tracking pixel, but–"

"Yeah, do that, the tracking pickle thing."

The other is that the "did they open this?" feature was rolled out purely for metrics knowing that it's imprecise, and later on got repurposed for something unsuitable without looking at how the "did this email get opened?" facility actually worked.

I would definitely open my mail if it came with a tracking pickle.