The IRS will throw a wide net, and see who fail to wiggle out. I have been subpoenaed merely because I provide payroll software for others to use, and the case had nothing to do with payroll (it was tax prep fraud, where the preparer was faking returns after the fact, and collecting refunds in the names of their clients). The IRS found my software on one of the computers...
There are those who are paid to really manage payroll, and thus, openly accept liability. They usually do nothing without first having the funds available, deposit every payroll, and properly limit exposure. There are those who only do calculations and provide unsigned reports, sort of a grey market. Then there are the rest, who likely do not realize their liability.
Whether or not someone is eventually held liable, defense is not free!