This is a question I had to wrestle with when I was a professional pastor. I resigned because I came to the conclusion that the bible does not support the practice of pastors receiving salaries. Why, then, do so many churches continue this practice? (This question comes from my prior post Sorry, But I Can't Stop Asking Questions).
Five reasons stand out to me as to why this keeps happening:
1. Church history. Roman Catholic priests received payment of various sorts. The Reformers kept this practice, as they did many other Roman aspects of church life.
2. Misinterpretation of scripture. Nowhere in the New Testament is it said that pastors should receive regular salaries. The I Corinthians 9 passage refers to traveling evangelists, not pastors. As for I Timothy 5, "double honor" may or may not be talking about money. However, if it is then it's referring to love gifts after the fact as opposed to salaries given beforehand.
3. People think they need an "expert." The folks in the pews want a person to come in from the outside who has attended seminary. He's given a salary to preach, administer, and coordinate the worship ceremonies.
4. Pastors like it. I can say from experience that it is nice to receive a paycheck for studying the bible and reading theological books much of the time. Why would pastors ever speak against this? Instead, what they do is perpetuate it.
5. "It's what we've always done." Churches have been paying salaries to pastors for so long that many people cannot imagine any alternative.
The above five reasons are not exhaustive. Churches have their combination of reasons for handing out salaries to pastors. The big problem is that they have no scriptural support for doing so.