Well they are only available to FFLs with a Class 3 or Class 7 SOT who have a demo request from an LEA on department letterhead. Those letters are getting increasingly more difficult to get. The ATF also has to approve the transfer.
They are so cheap because the number of potential legal purchasers is so small.
PS. SOT 7s may also purchase these weapons from SOT 7s and SOT 3s who are going out of business. These weapons may not be held without a SOT and accompanying business.