The difference in complying and waiting for a subpoena - due process. First off, it has not been established by law that a Senator or Congressman has ANY legal right to demand this documentation.
It is like if a police officer came to my home and wanted to search inside, with no warrant. I am a law-abiding citizen, I don't think there would be anything illegal found in my home. I would still refuse (depending on circumstance - if I was told that a neighbor saw someone breaking in to my home while I was out - it would be reasonable to have the police check it out), The law says - no warrant - I do not have to allow the search. Now - the police do have the right to have an officer stay outside so that I cannot dispose of evidence, and they can radio in for a warrant. If a judge or magistrate grants the warrant, legally I must permit the search.
In the position of any of these ministries, I would at the very least refuse the request until my own legal counsel gave advice on the legality of the request. I tend to believe that the IRS has the professionals, the previous experience, and the ability to detect fraud (at least on any fairly large scale), I would also anticipate that the IRS will, in fact, audit at least some of the ministries involved (if for no other reason than to avoid any appearance that they are not doing their job.) If you do not stand up for your rights, they are automatically given up - they may well get stomped on even if you do stand up.
I'm sorry, but being a Senator or Congressman does not automatically give one the experience and in depth knowledge of the US Tax code to audio someone's return - the code is so complex that even two IRS agents often disagree about what parts of it mean.
Of course, every one of these ministries may be absolutely totally in accord with all of the US law - and still not be handling things in the manner that the Father, Son, and Holy Spirit approve. But that question is not likely to be answered until the day that every knee bows, and it most certainly is not within the authority of the US Congress.