It's strange that people tend to expect expert advice for free from friends, while they don't expect free samples of physical products. Why should they be any different? In both cases, these are professional activities that people do for a living.
When asked, people clarify this distinction by explaining that physical products cost the supplier something to produce, making these professional favors unsustainable. For some reason, this mentality does not carry over to service based activities. People expect free legal counsel, medical opinions, and advice on how to make their iOS apps as part of friendship, in this case being loosely defined.
People should value the time of an expert and cost of goods sold in similar ways. In other words, ask for help only when you really need it. Don't abuse this privilege and don't expect it of acquaintances.