It's a question of emphasis. Suppose someone commits murder. A judge of the murderer can be optimistic or pessimistic. The judge can emphasize the ideals that the murderer failed to live up to, or she can focus on the reality of what the criminal did.

With their abstract models, economists have focused on the perfect, optimal form of capitalism, thus indirectly associating all real capitalist economies with approximations of that ideal. All of which easily gets conflated with an ideal in the normative sense, because we're talking about social rather than natural systems.

The classical economists praised capitalism on individualist, liberal grounds, because economics for them was still philosophical. When economists thought of themselves more as professional scientists, they couldn't get away with issuing such bald value judgments, so they publicly praise capitalism more by insinuation.