"Don't muzzle the ox" 1 Corinthians 9:9

For it is written in the law of Moses, thou shalt not muzzle the mouth of the ox that treadeth out the corn. Doth God take care for oxen? (KJV)

Then: There's a double-whammy here. (1) Even the law teaches that a laborer is worthy of being paid (allowed to eat). (2) The law moves into the law of nature, the animal kingdom, showing that God himself cares for them by ensuring they have food to eat. Further, men who take animals from the wild and use them must be sure they have food for sustenance. The original law is in Dueteronomy 25:4. Fee (p. 408) asserts that Paul uses the law because of its principle and is not at all interested in the literal content, only its application. That is reasonable.

Now: There are an infinite number of regulations and laws to ensure that no one takes advantage of laborers. Some even apply to ministers.

Extra source: The First Epistle to the Corinthians by Gordon D. Fee (Eerdman's 1987) online http://books.google.com/books?id=XlBp10nUTXAC&dq=Gordon+Fee+Bible+commentator&printsec=frontcover&source=in&hl=en&ei=SwLNSu3nA4v6Mf2XlDo&sa=X&oi=book_result&ct=result&resnum=12#v=onepage&q=&f=false