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When a guardian becomes custodian over a vulnerable old person and put them in assisting facilities they can decide about the selling of assets of those people on behalf of their wards; but it is always open to abuse.
Is there any laws preventing such an abuse in the US?
Such laws are ubiquitous. An example is Washington's RCW 11.130.325. The nutshell version is:
(1) A guardian for an adult is a fiduciary and owes the highest duty of good faith and care to the person under a guardianship. The guardian shall not substitute his or her moral or religious values, opinions, or philosophical beliefs for those of the person under a guardianship. Except as otherwise limited by the court, a guardian for an adult shall make decisions regarding the support, care, education, health, and welfare of the adult subject to guardianship to the extent necessitated by the adult's limitations.
(2) A guardian for an adult shall promote the self-determination of the adult and, to the extent reasonably feasible, encourage the adult to participate in decisions, act on the adult's own behalf, and develop or regain the capacity to manage the adult's personal affairs.
Then, under §350,
(1) The court may remove a guardian for an adult for failure to perform the guardian's duties or for other good cause and appoint a successor guardian to assume the duties of guardian
Being "predatory" i.e. acting in one's own interest is good cause.
