The party collecting the tax is presumed to hold it in trust for the benefit of the government to whom it is due.
A trust-fund tax is a type of tax or debt where (absent a personal guarantee) the management or responsible employees of a corporation or other entity with limited liability can be held personally liable for its non-payment.
Thus, in effect, failing to pay a trust-fund tax is considered the equivalent of management embezzling the funds that were due, and being personally liable for non-payment.
In California, for example, it is a misdemeanor for the management of a business entity to willfully fail to remit trust-fund taxes such as any withholding from an employee's wages made pursuant to a state or local law, and if the amount of tax due exceeds $500 the class of crime becomes a felony (see California Labor Code Section 227).
On the other hand, income tax evasion is punishable with a fine and/or imprisonment (see California Revenue and Taxation Code section 19706).