I never undersstood how poor or low middle class people donate to church and the church willingly accepts.
Maybe they think if they donate to their church, "God" will bless with them millions of dollars in return. 
That is very common.
Look up Prosperity Theology.
I will copy and paste some short bits from wikipedia, but there's a lot more out there on it.
Prosperity theology (sometimes referred to as the prosperity gospel, the health and wealth gospel, or the gospel of success)[A] is a Christian religious doctrine that financial blessing is the will of God for Christians, and that faith, positive speech, and donations to Christian ministries will increase one's material wealth. Based on non-traditional interpretations of the Bible, often with emphasis on the Book of Malachi, the doctrine views the Bible as a contract between God and humans: if humans have faith in God, he will deliver his promises of security and prosperity. Confessing these promises to be true is perceived as an act of faith, which God will honor.
The doctrine emphasizes the importance of personal empowerment, proposing that it is God's will for his people to be happy. The atonement (reconciliation with God) is interpreted to include the alleviation of sickness and poverty, which are viewed as curses to be broken by faith. This is believed to be achieved through visualization and positive confession, and is often taught in mechanical and contractual terms.
It was during the Healing Revivals of the 1950s that prosperity theology first came to prominence in the United States, although commentators have linked the origins of its theology to the New Thought movement which began in the 1800s. The prosperity teaching later figured prominently in the Word of Faith movement and 1980s televangelism. In the 1990s and 2000s, it was adopted by influential leaders in the Charismatic Movement and promoted by Christian missionaries throughout the world, sometimes leading to the establishment of mega-churches. Prominent leaders in the development of prosperity theology include E. W. Kenyon, Oral Roberts, A. A. Allen, Robert Tilton, T. L. Osborn, Joel Osteen, Creflo Dollar, Kenneth Copeland, Reverend Ike and Kenneth Hagin.
Churches in which the prosperity gospel is taught are often non-denominational and usually directed by a sole pastor or leader, although some have developed multi-church networks that bear similarities to denominations. Such churches typically set aside extended time to teach about giving and request donations from the congregation, encouraging positive speech and faith. Prosperity churches often teach about financial responsibility, though some journalists and academics have criticized their advice in this area as misleading.
Prosperity theology has been criticized by leaders in the Pentecostal and Charismatic movements, as well as other Christian denominations. These leaders maintain that it is irresponsible, promotes idolatry, and is contrary to scripture. Some critics have proposed that prosperity theology cultivates authoritarian organizations, with the leaders controlling the lives of the adherents. The doctrine has also become popular in South Korea; academics have attributed some of its success to its parallels with the traditional shamanistic culture. Prosperity theology has drawn followers from the American middle class and poor, and has been likened to the cargo cult phenomenon, traditional African religion, and black liberation theology.
