"We collect such commercial transactional messages so that we can better understand the behavior of the senders of such messages, and better understand our customer behavior and improve our products, services, and advertising. We may disclose, distribute, transfer, and sell such messages...." [emphasis mine]
Of course they might not sell your messages, but it's pretty weird that they'd put up a "we totally can sell your vaguely anonymized messages" and then not take advantage of it. What they sell to Uber may not be what they make available to you.
Even if they don't currently, they could start doing so at any time. And even if you somehow trusted them not to, they could get bought out by someone not so trustworthy.
Now I'm curious: what is the commercial value in the aggregated email content that would make someone want to pay for it, besides the purchase and receipt data that Slice is already providing (plus subscription and open rates)?
Lyft receipts provide origin and destination addresses as well as the exact date and time. That would be tremendously valuable for Uber. That info could be aggregated and then sold to Uber, or their privacy policy would also allow them to just scrub these e-mails (and the manner of scrubbing is not specified, so who knows if they consider the exact origin and destination locations to be personal information or not) and sell them to Uber directly.
"We collect such commercial transactional messages so that we can better understand the behavior of the senders of such messages, and better understand our customer behavior and improve our products, services, and advertising. We may disclose, distribute, transfer, and sell such messages...." [emphasis mine]
Of course they might not sell your messages, but it's pretty weird that they'd put up a "we totally can sell your vaguely anonymized messages" and then not take advantage of it. What they sell to Uber may not be what they make available to you.
Even if they don't currently, they could start doing so at any time. And even if you somehow trusted them not to, they could get bought out by someone not so trustworthy.