Yup.
The stamp says "ADMITTED" and has a place for "Class" and "Until" which is really for aliens; citizens are not "inspected" and "admitted" like aliens are, citizens are "examined" and allowed to enter. Consequently, the stamp really has no legal significance in a US Passport. I assume many inspectors stamp by habit even though the stamp means nothing.
Do they stamp US Passports at land POEs?
Incidence of stamping of US passports at land ports of entry in the US are far less frequent than stamping of such when using a port of entry facility at an airport.
Stamps in US passports have been used as evidence of presence in the US for legal purposes when dealing with the State Department, with the US Treasury/IRS and the FBI. Probably more than that too.