One explanation is that movie patrons are considering their total willingness to pay for the movie experience so that if the ticket price plus the market price of popcorn is less than their willingness to pay (WTP), the theater has an opportunity to extract more consumer surplus by charging higher than market prices for the popcorn (that is, price discrimination).
There is a working paper on the subject by Gill and Hartmann (2008), the abstract of which reads:
Prices for goods such as blades for razors, ink for printers and concessions at movies are often
set well above cost. Theory has shown that this could yield a profitable price discrimination
strategy often termed “metering.” The idea is that a customer’s intensity of demand for
aftermarket goods (e.g. the concessions) provides a meter of how much the customer is willing to
pay for the primary good (e.g. admission). If this correlation in tastes for the two goods is
positive, a high price on the aftermarket good allows firms to extract a greater total price
(admissions plus concessions) from higher type customers. This paper develops a simple
aggregate model of discrete-continuous demand to motivate how this correlation can be tested
using simple regression techniques and readily available firm data. Model simulations illustrate
that the regressions can be used to predict whether aftermarket prices should be above, below or
equal to their marginal cost. We then apply the approach to box-office and concession data from
a chain of Spanish theaters and find that high priced concessions do extract more surplus from
customers with a greater willingness to pay for the admission ticket.
Locay and Rodriquez (1992) make a similar argument in a JPE article. They essentially argue that purchases of things like movie tickets are made by groups; once individuals are constrained by the group's choice, the firm has additional market power:
We present models in which price discrimination in the context of a
two-part price can occur in some competitive markets. Purchases take
place in groups, which choose which firms to patronize. While firms
are perfectly competitive with respect to groups, they have some
market power over individual consumers, who are constrained by their
groups' choices. We find that firms will charge an entry fee that is
below marginal cost, and the second part of the price is marked
up above marginal cost. The markup not only is positive but
increases with the quality of the product.
The quote you are looking for is similar, and again attributes the discrepancy to price discrimination. From the Armchair Economist (p. 159):
The purpose of expensive popcorn is not to extract a lot of money from
customers. That purpose would be better served by cheap popcorn and
expensive movie tickets. Instead, the purpose of expensive popcorn is
to extract different sums from different customers. Popcorn lovers,
who have more fun at the movies, pay more for their additional
That is, some people like popcorn more than others. The latter idea is that the movie experience for popcorn lovers is worth more than the sum of its parts: that a movie ticket + popcorn is worth more than either of them separately for some people.