Legal Question in Criminal Law in Washington

They can.

"The affirmative right to detain shoplifters derives from the common law right of citizen arrest. State v. Gonzales, 24 Wash.App. 437, 604 P.2d 168 (1979), review denied, 93 Wash.2d 1028 (1980)." State v. Miller, 103 Wash. 2d 792, 795, 698 P.2d 554, 556 (1985)

Further, by statute, they can't be sued or criminally prosecuted as long as there was reasonable grounds to believe that a shoplift was occurring.

RCW 9A.16.080

Action for being detained on mercantile establishment premises for investigation—"Reasonable grounds" as defense.

In any criminal action brought by reason of any person having been detained on or in the immediate vicinity of the premises of a mercantile establishment for the purpose of investigation or questioning as to the ownership of any merchandise, it shall be a defense of such action that the person was detained in a reasonable manner and for not more than a reasonable time to permit such investigation or questioning by a peace officer, by the owner of the mercantile establishment, or by the owner's authorized employee or agent, and that such peace officer, owner, employee, or agent had reasonable grounds to believe that the person so detained was committing or attempting to commit theft or shoplifting on such premises of such merchandise. As used in this section, "reasonable grounds" shall include, but not be limited to, knowledge that a person has concealed possession of unpurchased merchandise of a mercantile establishment, and a "reasonable time" shall mean the time necessary to permit the person detained to make a statement or to refuse to make a statement, and the time necessary to examine employees and records of the mercantile establishment relative to the ownership of the merchandise.

RCW 4.24.220

Action for being detained on mercantile establishment premises for investigation—"Reasonable grounds" as defense.

In any civil action brought by reason of any person having been detained on or in the immediate vicinity of the premises of a mercantile establishment for the purpose of investigation or questioning as to the ownership of any merchandise, it shall be a defense of such action that the person was detained in a reasonable manner and for not more than a reasonable time to permit such investigation or questioning by a peace officer or by the owner of the mercantile establishment, his or her authorized employee or agent, and that such peace officer, owner, employee, or agent had reasonable grounds to believe that the person so detained was committing or attempting to commit larceny or shoplifting on such premises of such merchandise. As used in this section, "reasonable grounds" shall include, but not be limited to, knowledge that a person has concealed possession of unpurchased merchandise of a mercantile establishment, and a "reasonable time" shall mean the time necessary to permit the person detained to make a statement or to refuse to make a statement, and the time necessary to examine employees and records of the mercantile establishment relative to the ownership of the merchandise.