It's not illegal because the US isn't interpreting this as a military coup. As far as I can tell an "unlawful decision" is not a part of the law and it seems awfully vague-while international organizations may do this, the US probably avoids ruling on the laws of other nations.
To be a military coup, the military would need to be running the Ukraine, which it is not, so cannot fulfill this definition.
A coup of any kind is always suspect. The intent of the law is to prevent misuse of funds specifically if they will go towards human rights abuses and line the pockets of dictators.
coup d'état (/ˌkuːdeɪˈtɑː/; French: blow of state; plural: coups d'état), also known as a coup, a putsch, or an overthrow, is the sudden deposition of a government,1 usually by a small group of the existing state establishment—typically the military—to depose the extant government and replace it with another body, civil or military.
The Ukrainian revolution here was not sudden, was not by a small group, or by an existing part of the state establishment. It is not a coup is any sense. It is a popular uprising.
The new Ukrainian government is working with Organization for Security and Cooperation in Europe to work out peaceful and constructive solutions to the crisis and ensure the human rights of the people of the Ukraine. It doesn't seem to be a huge concern that the economic support sent to them will be used for torture, disappearing opposition, illegal detention, or other such severe human rights abuses that the US attempts to avoid helping support when coups occur.
Russia is arguing that they are a small group of dangerous Neo Nazis that want to kill Russians and Jews, of course, that have no connection with the Orange Revolution, but due to the fact that the Kiev government is cooperating with international monitors, I feel confident the people of Ukraine are safe from ethnic genocide
Congress will review the legality of the transfer of the funds and may choose to stop them at any time. Decisions are reversed due to human rights abuses. I'm not sure about legal actions a citizen should take if they disagreed with the economic help, but you could write the members of House Foreign Affairs Committee.
While the US routinely funds human rights abusers, although the law allows it if there are greater benefits than risks, which is sometimes interpreted as an open license, I can find only one example of the US ever breaking this part of the law, and it is currently under consideration of cutting off aid to them (Porfirio Lobo in Honduras). In other words, there seems to be pretty good consistency in following this portion of the law.