There are plenty of reasons to prefer secure browsing by default. Perhaps I subscribe to feeds that would be flagged as "suspect" by programs like PRISM[1] and XKeyscore[2], and would like to make the NSA work at least a little bit to find that out.
And you can pay for that feature. I have no real stake in the game here, but I'm failing to see why that is a must-have feature for every user of the service.
Feasibly, the lack of HTTPS for a subset of users could be used as an attack vector against Feedly, which could surface information about me that I do not want revealed even if I were a Pro user using HTTPS.