Well, I must admit that I had not gone back and read Judge Hand's decision in detail. I had (incorrectly) assumed that it was a Supreme Court decision, not merely an Appellate Court one, and therefore that it would have the level of argumentation and support that are typical of SC cases.

So the only support that this gives is citing US v. Isham and Bullen v. Wisconsin. So, perhaps to avoid being guilty of presuming twice in one thread, I went looking for Isham. I found it at https://supreme.justia.com/cases/federal/us/84/496/

It is a Supreme Court decision, and contains the following text:

"It is said that the transaction proved upon the trial in this case is a device to avoid the payment of a stamp duty, and that its operation is that of a fraud upon the revenue. This may by true, and if not true in fact in this case, it may well be true in other instances. To this objection there are two answers:

"1st. That if the device is carried out by the means of legal forms, it is subject to no legal censure. To illustrate. The Stamp Act of 1862 imposed a duty of two cents upon a bank-check, when drawn for an amount not less than twenty dollars. A careful individual, having the amount of twenty dollars to pay, pays the same by handing to his creditor two checks of ten dollars each. He thus draws checks in payment of his debt to the amount of twenty dollars, and yet pays no stamp duty. This practice and this system he pursues habitually and persistently. While his operations deprive the government of the duties it might reasonably expect to receive, it is not perceived that the practice is open to the charge of fraud. He resorts to devices to avoid the payment of duties, but they are not illegal. He has the legal right to split up his evidences of payment and thus to avoid the tax. The device we are considering is of the same nature.

"Another answer may be given to the objection more comprehensive in its character. It is this: that the adoption of a rule that the form of the instrument can be disregarded, and its real character be investigated for the purpose of determining the stamp duty, would produce difficulties and inconveniences vastly more injurious than that complained of. Such a rule would destroy the circulating capacity of bills, or drafts, or orders. The present act imposes the same stamp duty upon inland bills of exchange and promissory notes, but this is an accidental circumstance only. Suppose that the draft is made subject to a tax of five cents on the hundred dollars, and the note to a tax of ten cents on the hundred dollars. The defendant contends that a draft or bill drawn by one officer of a company upon another officer of the same company is, in legal effect, a promissory note. Upon the supposition thus made, its real character would require a tax of twice the amount of that indicated upon its face, and if the stamp be too small, the instrument is absolutely void from its inception. [Footnote 11] In the language of the statute, it shall be "deemed invalid and of no effect."

"Is every man to whom a paper in the form of a bill of exchange is presented bound to inquire whether there are not outside circumstances that may affect its nature? Having ascertained this, is he bound to delay all proceedings until he can take legal advice upon its nature and character? This he must do upon the theory contended for, and he must be certain also that his advice is correct; otherwise he will lose the money he advances upon the bill. The same rule, it is contended, will apply where the drawee does not appear upon the face of the bill to be an officer of the company. Such is the case before us, where Mr. Canda, the drawee, does not appear upon the bill itself to be connected with the company, and yet the prosecution contends that it may be proved that he is its treasurer, and that thereupon the instrument ceases to be a draft or order for the payment of money and becomes a promissory note.

"That the rule contended for is impracticable in a commercial country is too obvious to require farther illustration. We are satisfied that the principles heretofore laid down must govern the case before us."

End of quote from Isham.

So the argument is not that it is moral to avoid taxes. The argument is that it is infeasible for the government to judge all transactions as to whether they are intended to avoid taxes or not, and so the only thing to do is to apply the tax rules as written.

I'm not sure where that leaves us. There's argument and reasoning supporting the position, if you like, but it's not the argument that I think either of us expected. (Or maybe it is what you expected. Maybe that's what you meant by "only supporting the legality, not the morality"...)

I agree that Judge Hand did not explain why his claim was morally correct. (In fact, we shouldn't expect it - an appellate judge doesn't rule on "moral", but only on "legal". One could view the quote as saying there is no legal patriotic duty to pay more tax.)