This is all necessarily speculative, we might never have sufficient visibility to know all the facts.
I'm merely attempting to provide the strongest reply the administration could provide if they cared to try. I believe it's reasonably grounded in facts.
1. US government openly does not recognize Maduro as the legitimate head of state of Venezuela
2. US government does recognize Edmundo González Urrutia as the president-elect.
3. Venezuelan opposition has been heavily lobbying in an effort to get foreign governments to intervene in Venezuela
All of these things are verifiable facts, I think they can be distilled into my perfectly reasonable suggestion as to how the US could fend off such criticism.
Unilateral action by the US against a souvenir nation should be criticized regardless the nation.
There's no second party to this action, it's the US's alone. Even if we accept the electoral fraud claims, Venezuela did not ask for US intervention. The rightfully elected leader of a nation can't call for a second nation to invade and bomb their nation.
Why not?
Decided by whom?
International law, also known as public international law and the law of nations, is the set of rules, norms, legal customs and standards that states and other actors feel an obligation to, and generally do, obey in their mutual relations. In international relations, actors are simply the individuals and collective entities, such as states, international organizations, and non-state groups, which can make behavioral choices, whether lawful or unlawful. Rules are formal, typically written expectations that outline required behavior, while norms are informal, often unwritten guidelines about appropriate behavior that are shaped by custom and social practice.[1] It establishes norms for states across a broad range of domains, including war and diplomacy, economic relations, and human rights.
Were they? And is that the justification the US has cited? If not, you're writing fan fiction and that's not really interesting.
I'm not a supporter of totalitarian regimes including Maduro's, but the US has a track record of producing very poor outcomes for people in South America when they topple one leader in favor of a more--shall we say--"market friendly" character waiting in the wings.
As for international law, it is extremely clear, prohibiting the threat or use of force against the territorial integrity or political independence of any state. International law recognizes only two clear exceptions: self defense or a US Security Councul resolution.