[Edit] I guess all of this misses the original point I think you were making that it is easy to accidentally commit a felony. I will grant you that in its entirety. But I have yet to be given any data that suggests this is happening regularly (eg daily) for normal people. Cheating on your taxes (which we will suppose everyone in the US is doing for the sake of argument) happens 1x per year. How are we going to fill in the remaining 364 felonies we need? Not following ToS in some way that breaks the CFAA?
[Original comment]
The Cesarini case was taxed under the 1939 tax code "Section 22(a)", which according to the ruling held that "[p]erhaps a more appropriate interpretation of Section 22(a) would be to hold that all windfalls ... are taxable income under its sweeping language. ... Insofar as the policy of Section 22(a) is to impose similar tax burdens on persons in similar circumstances, there is no basis for distinguishing value received as windfall and ... value received as salary."
Interestingly they also made the argument that it should count as a gift which the court rejected.
https://law.justia.com/cases/federal/district-courts/FSupp/2...
Afaict, that isn't true of the modern tax code (Title 26 - https://www.law.cornell.edu/uscode/text/26), which the plantiffs tried to argue (unsuccessfully) they should be taxed under: https://www.law.cornell.edu/uscode/text/26/6501
(A) General rule If the taxpayer omits from gross income an amount properly includible therein and—
(i) such amount is in excess of 25 percent of the amount of gross income stated in the return, or
(ii) such amount—
(I) is attributable to one or more assets with respect to which information is required to be reported under section 6038D (or would be so required if such section were applied without regard to the dollar threshold specified in subsection (a) thereof and without regard to any exceptions provided pursuant to subsection (h)(1) thereof), and
(II) is in excess of $5,000,
the tax may be assessed, or a proceeding in court for collection of such tax may be begun without assessment, at any time within 6 years after the return was filed.
So assuming you don't find 5k, 25% of your gross income, or any amount of money in foreign financial assets/stocks/bonds, it shouldn't matter.
Arguably this is at odds with the broadest definition of gross income, ie "including (but not limited to)":
https://www.law.cornell.edu/uscode/text/26/61
I don't think its at all clear that you need to pay taxes on gross income from found monies under the limits above. And knowingly evading is a prerequisite to felony tax fraud; which is to say there is a mens rea component. I would still report if I found a significant amount of money just to avoid the possibility of future audits, but I don't think you can reasonably argue that people who aren't reporting a 20$ bill they found are knowingly evading taxes.