

The doctrine is called force majeure. Most contracts have a force majeure clause.
If an external factor makes a contract impossible as agreed, the contract can be made void under force majeure. This is very common, and suddenly applied tariffs would likely be covered by a force majeure clause because neither party were responsible for them.
The rich have more assets and income sources and have staff who do a bunch of financial chicanery to try to hide their tax evasion. Auditing the rich requires significantly more work than the poor.
These decisions are not about doing what makes sense. The IRS is a revenue generator. Spending money on the IRS brings in significantly more money in unpaid taxes than it costs.
The goal is not to do what’s best for the government’s budget. Its really about ensuring that their criminal friends can continue to get away with it.