The point here is that even learning about the problem is costly for the household. For small savings, a lobbying group will have to convince its contributors very quickly. Suppose we knew the names and addresses of the 10 million affected households. Recipients of the letters will reasonably question the trustworthiness of the solicitation. The chances of getting any more than a small return from this kind of solicitation is highly unlikely.
Free riding occurs when someone enjoys the benefits of something without paying for it.
The lobbying effort, if successful, will benefit all blue jean consumers regardless whether they contribute to the lobbying campaign or not. In economic terms we say that the lobbying effort is a public good because individual households cannot be excluded from the benefits of successful lobbying. Even if one person or group decided to spearhead the effort and collect contributions from others, the difficulties they would face would likely be insurmountable. In the end, government decision makers would probably hear very little in the way of opposition to a proposed tariff.