Yes, it is almost certainly legal. An employer may determine the duties of  employees at will, subject only--1) Any employment contracts, including union agreements--if there are such contracts, any terms of theirs setting or delineating duties are enforceable;2)  Any safety regulations over  your particular industy or job, which may specificy that personal with certain training or qualifications conduct safety checks--but this would only apply to a few industries or jobs, such as certain  medical, aviation, etc. fields.Otherwise, your employer may make this part of your job.