You actually can do that. It's only "legal tender for all debts", meaning that a restaurant would be required to accept the pennies after providing service, but my local bus line(that takes payment upfront) won't let you on if you try to give them pennies.
https://www.treasury.gov/resource-center/faqs/Currency/Pages...
And every ecommerce site out there doesn't allow USD, but only a derivative currency convertible to USD on-demand with complex rules attached.
Never heard of this. Care to elaborate? Are you just talking about PayPal?
"The pertinent portion of law that applies to your question is the Coinage Act of 1965, specifically Section 31 U.S.C. 5103, entitled "Legal tender," which states: "United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues."
This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no Federal statute mandating that a private business, a person or an organization must accept currency or coins as for payment for goods and/or services. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise. For example, a bus line may prohibit payment of fares in pennies or dollar bills. In addition, movie theaters, convenience stores and gas stations may refuse to accept large denomination currency (usually notes above $20) as a matter of policy."
https://www.treasury.gov/resource-center/faqs/Currency/Pages...
The concerning thing about bitcoin, and any cryptocurrency is that by nature, the larger it gets (as in total outstanding coins), the more difficult it is to create more currency through reconciliation. With traditional currencies, it's relatively easy to print money. Printing money is done mainly to keep an agreed upon healthy inflation rate of around 2%. Why this is important is when deflation happens, loans are no longer attractive because it's always better to wait because items increase in value relative to the currency. In other words, why would I borrow $1000 today when that same loan amount would buy $2000 worth of stuff next week. When people stop borrowing, it has nasty economic implications. (Money Multiplier effect and all that).
That's probably a horrible explanation by me, but the short version is crypto, since it's harder to create over time leads to a natural deflation that increases over time.
FWIW: I don't know a whole lot about crypto, but the things mentioned are concerning to me. I mean it might be a good time to follow the herd so to speak, as long as you know there will probably come a time when the herd runs off a cliff.