There is similar language in the listing agreements we use (approved by the SC Realtors®). Some companies use their own forms; we use the SC forms. The seller must check the appropriate box to indicate whether they agree to have a lockbox installed. The next paragraphs says:

"Owner acknowledges and agrees that neither Broker, nor Broker's agents, subagents, or anyone showing the property through the MLS, shall be responsible for any damage to, or loss of personal property, or to the realty, except such damage or loss as may be caused by the negligence of such party. Owner further acknowledges that Broker nor MLS is an insurer against the loss of personal property and agrees to release Broker and MLS from any responsibility therefore."

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If you read the listing agreement that you have sellers sign, you may find a section with language such as below for the client to sign off on.

"Owner is advised that, even though the use of the “Key Box" will probably increase the number of showings, the use of the “Key Box" involves risk of unauthorized entry to the property, theft or damage to the home and personal property therein, as well as the risk of personal injury to the occupants of the property.
Owner, with knowledge of the risks involved in the use of the “Key Box," hereby authorizes REALTOR® FIRM and Associate to place a “Key Box" on the home to facilitate the sale/lease of the property.
Owner agrees that neither REALTOR® FIRM, Associate of REALTOR® FIRM, other MLS Participants, nor the Albuquerque Metropolitan Board of REALTORS® shall be liable for any damages or claims arising out of the authorized or unauthorized use of the “Key Box" or its key and agrees to indemnify and hold harmless REALTOR® FIRM and Associate, other MLS Participants, and the Albuquerque
Metropolitan Board of REALTORS® for any damages, claims, or liability arising therefrom, including, but not limited to damages, claims, or liability resulting from loss or damage to personal or real property or personal injury, costs, and attorney's fees. Owner hereby assumes the full risk of any such loss, damage, or injury. Owner agrees to maintain insurance covering personal injury and property damage to the aforesaid real or personal property and any person therein, and acknowledges and understands that his current homeowner's policy may not provide such coverage arising out of the use of a “Key Box" or a “Key Box" key or may not provide such coverage on vacant premises."