This is mainly to ensure data integrity, and to prevent any tampering of data.
In what way does a MAC not perform this task? A signature binds a person to a document, that's
rather different. (Well it need not be a person, its usually a certification authority).
A requirement is not the same thing as a technique. The requirement is to prevent tampering,
not the technique chosen to satisfy the requirement. Sounds like you've been ordered to
do something a particular way, not given a set of functional requirements!
Asymmetric crypto with public key management infrastructure is much more heavyweight than
shared-secret crypto, and is not normally doable on a tiny microcontroller. Yes, that means
a microcontroller is more limited in its options, and that you'd better have distinct shared
secrets for each client.