Perhaps because J.P. Morgan is an investment bank and this was technically the sale of a security on which they routinely make a profit, the sale wasn't flagged as a one-time charge like it would have been had they sold a wholly owned subsidiary. But apparently the more astute analysts considered it more like the sale of a subsidiary than a stock trade. Also, as Mr. Abbot points out, there's now a hole in the future revenue stream. The earnings from the bank that was sold will need to be replaced. Perhaps J.P. Morgan has an investment in mind that will do that, but uncertainty is never good for stock prices.
Banks always set aside cash from earnings to cover loan defaults.
Basically they have to guess at how much they will need. When economic times improve, as they have, the bank may find that it has set aside too much cash. Normally a bank will fix that problem by simply reducing or eliminating the loan provision set aside for a few quarters until things are back in balance. In effect, they shifted income from last year to this year. Especially when the money came out of a year when the stock was in free fall anyway and was added to a year where it was important to show a strong recovery.