was rechtfertigt es einen ... Präsidenten ( z.B. Trump) alle Urteile ... aufzuheben und sich sogar selbst freischwebend davonzustehlen?

Zitat gekürzt, der Sachlichkeit wegen

Article 2, Section 2, Clause 1 besagt

The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices, and he shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment.

Das Recht ist limitiert

Federal pardons issued by the president apply only to federal offenses; they do not apply to state or local offenses or private civil offenses. Federal pardons also do not apply to cases of impeachment. Pardons for state crimes are handled by governors or a state pardon board. ^[5]

One limitation to the president's power to grant pardons is "in cases of  impeachment." This means that the president cannot use a pardon to stop an officeholder from being impeached, or to undo the effects of an impeachment and conviction.

Federal pardons in the United States - Wikipedia

Bei uns ist das ähnlich geregelt

Similar to the United States, the right to grant pardon in  Germany is divided between the federal and the state level. Federal jurisdiction in matters of criminal law is mostly restricted to appeals against decisions of state courts. Only "political" crimes like  treason or  terrorism are tried on behalf of the  federal government by the highest state courts. Accordingly, the category of persons eligible for a federal pardon is rather narrow. The right to grant a federal pardon lies in the office of the  President of Germany, but the president can transfer this power to other persons, such as the chancellor or the minister of justice.

Pardon - Wikipedia

und zwar gemäss Artikel 60 GG

Art. 60

(1) Der Bundespräsident ernennt und entläßt die Bundesrichter, die Bundesbeamten, die Offiziere und Unteroffiziere, soweit gesetzlich nichts anderes bestimmt ist.

(2)  Er übt im Einzelfalle für den Bund das Begnadigungsrecht aus.

(3) Er kann diese Befugnisse auf andere Behörden übertragen.

(4) Die Absätze 2 bis 4 des Artikels  46 finden auf den Bundespräsidenten entsprechende Anwendung.

Es ist somit nichts Ungewöhnliches.