You are right that the US government cannot do anything to these churches who fleece desperate people. Additionally, the FFRF has, so far, unsuccessfully sued the IRS over the "parsonage exemption" rules (TLDR: giving tax-free housing to "ministers of the gospel").

Where the FFRF has had success is with the IRS needing to resume audits for churches that violate the Johnson Amendment of 1954. This violation happens when tax-exempt churches endorse political candidates from the pulpit, which has been a growing issue since the "Pulpit Freedom Sunday" movement started in 2006 by Alliance Defending Freedom (a joint venture of every anti-gay and conservative Christian group you can think of). Unfortunately, even though the FFRF has so far successfully sued the IRS to enforce the Johnson Amendment of 1954, general US Congress fuckery in the last two years has prevented the IRS from enforcing its own rules. This makes FFRF's legal victory a moot point.

What should happen: If a church endorses a political candidate from the pulpit, it violates the Johnson Amendment of 1954. The IRS should revoke the church's tax-extempt status.

What has actually happened for the last 20 years or so: The IRS has ignored churches politicking from the pulpit.

Regarding the Johnson Amendment of 1954, the ADF loves to play the Christian Persecution Card™ and claims that the IRS wants to limit free speech from the pulpit (mainly to...surprise! raise funds). This is the backdrop for the "Pulpit Freedom Sunday" movement. In reality, the IRS does not want to limit free speech. Pastors can say whatever they want. However, if a church chooses to endorse a political candidate from the pulpit, that church loses its IRS tax-exempt status.