Carrying that amount of cash is a high-risk venture regardless of whether these questionable seizures are happening or not (e.g. the threat of theft). The other methods mentioned may cost a bit, but that cost goes towards lowering the risk of the transaction. For someone carrying 5-figures worth of cash, how can those costs be prohibitively expensive?
When it's all you have, and already allocated to an expense that's necessary in one way or another. The fact that cash is in paper form, and not electronic form doesn't imply that it's from the discretionary part of your budget.
Also, there is the phenomenon of people with low incomes opting out of the banking system for various reasons including distrust of it.
[edit: 11:24AM CDT added second paragraph]
1. delays have a low marginal cost
or
2. the receiving party is both wealthy and trusting enough to buffer the entire transaction
Banks operate on a principle of eventual consistency that lets transactions and accounts dangle in limbo for days to weeks. Delays cost money: lost opportunities, missed deadlines, rent. Is it really so hard to believe that the expected cost of a weeklong delay (which happen all the time) would be more than the expected cost of theft (which is rare in comparison)?
However, protecting yourself against something like this doesn't have to mean that you think it's a reasonable and justified action. As I explained earlier, for the same reason I generally wouldn't carry this much cash around, I don't leave valuable items inside my locked car, even though it is illegal and unjust for someone to break into it and steal them.