Earlier quoted context omitted.
The US did not agree to protect them. The signatures to the Budapest Memorandum agreed to respect Ukraine's sovereignty. Of the signatories, Russia is the only one that has violated the agreement.
Are you sure about that? Wikipedia says the following: " 3. Refrain from economic coercion designed to subordinate to their own interest the exercise by Ukraine, the Republic of Belarus, and Kazakhstan of the rights inherent in its sovereignty and thus to secure advantages of any kind. 4. Seek immediate Security Council action to provide assistance to the signatory if they "should become a victim of an act of aggress…
https://treaties.un.org/doc/Publication/UNTS/Volume%203007/P...