You can read Hayes' message to Congress upon his veto here. Essentially, his justification was that the Act changed the terms of the "Burlingame Treaty" of 1869, a treaty between the United States and China negotiated by Anson Burlingame, the U.S. Ambassador/Envoy to China at that time. Hayes believed it was outside the authority of Congress to change a treaty unilaterally. It was, instead, a power of the President to renegotiate the terms of the treaty directly with the reciprocal country.

In his message to Congress, Hayes stated that there was "peace and friendship" between the two countries, and that they should not "insult or oppress each other for any trifling cause" that may "produce an estrangement between them". By changing the terms of the treaty, the Act might lead to conflict between the two countries. Therefore, he decided it was necessary to veto it.

He wrote the same thing in his private diary: "As I see it, our treaty with China forbids me to give it my approval." He goes on to explain that the treaty was sought by the United States, not by China, and approved by Congress. If not illegal, it would certainly be unjust to change the terms by way of an act of Congress, without input from China: "One of the parties to a treaty cannot rightfully by legislation violate it."

Critics and partisans did not entirely take Hayes' explanation at face value. The Sacramento Record Union newspaper, for example, attributed his veto to Hayes being a devout Methodist, and was taking direction from the clergy, who had missions in China and were trying to spread the gospel in the East.

The States Rights Democrat of Albany, Oregon, agreed, and added that Hayes was acting like a typical "New England bigot", insinuating he was motivated by a prejudicial attitude toward Catholics and Catholic immigrants, who overwhelmingly voted Democratic in that era. (A ban on Chinese immigration was seen as most beneficial to immigrant labor from Europe, who were predominantly Catholic in the late 19th century.) Hayes was from Ohio, but he attended law school at Harvard, and he had many relatives in New England. His ancestry was of old New England Puritan stock; his father had immigrated to Ohio from Vermont, though died before Hayes was born.

The Morning Appeal of Carson City, Nevada, believed Hayes was motivated by an "over-weening admiration for the Commercial Class whose sympathies and tastes so commend themselves to his love of gain". In other words, it was all about money. The accusation was that Hayes had aligned himself with the interests of big business, the railroad magnates, and the "robber barons", who would have liked to keep the ability to hire cheap immigrant labor from China. American businessmen who dealt in Chinese imports and exports, too, might find themselves at a disadvantage if China retaliated against the act in any way.

There is a at least a nugget of truth in these accusations. In Hayes' diary, he wrote that breaking the Burlingame Treaty could threaten American "traders, missionaries, and travelers" in China, either financially or through their immigration status.

Hayes' decision certainly wasn't out of a concern for the plight of Chinese immigrants, or out of any sense of equality. From Hayes' diary entry dated February 23, 1879, a little over a week before he vetoed the bill:

"Both houses have passed a bill intended to prevent Chinese from coming to this country in large numbers. I am satisfied the present Chinese labor invasion (it is not in any proper sense immigration — women and children do not come) is pernicious and should be discouraged. Our experience in dealing with the weaker races — the negroes and Indians, for example, — is not encouraging. We shall oppress the Chinamen, and their presence will make hoodlums or vagabonds of their oppressors. I therefore would consider with favor suitable measures to discourage the Chinese from coming to our shores. But I suspect that this bill is inconsistent with our treaty obligations. I must carefully examine it. If it violates the National faith, I must decline to approve it."

After the veto, he wrote in his diary that the criticism of the veto was severe, even though he agreed that Chinese immigration was "hateful". Chinese immigration negatively affected the fortunes of American laborers and threatened the safety of the United States:

"The veto of the anti-Chinese bill is generally approved east of the Rocky Mountains, and bitterly denounced west of the mountains. I was burned in effigy in one town! No doubt a population without women — without wives and mothers — that can't assimilate with us, that underbids our laborers, must be hateful. It should be made certain by proper methods that such an invasion cannot permanently override our people. It cannot safely be admitted into the bosom of our American society."