Why is glycine considered a nonpolar amino acid but a polar molecule?
The first part of your question illustrates a common confusion of beginners between the physiochemical properties of free amino acids in solution, and the properties of that part of an amino acid that remains after it has participated in a condensation reaction to form part of a polypeptide. They are different. (It was for that reason that I modified the question to make it clear that the ‘consideration’ of glycine as nonpolar applies to it in its protein context.)

I have prepared a diagram to clarify.


As can be seen on line 1, all free amino acids at physiological pH are charged because of the pKa of the  α-amino and α-carboxyl groups (proline, as an imino acid is the exception). The chemical features common to all amino acids are shown in blue, for the second amino acid, and the unique feature — wherein amino acids differ — is shown in red.
To show amino acid 2 in the context of a polypeptide chain, I have condensed it with two other amino acids (1 and 3) to produce a tripeptide (line 3). (This is the situation most amino acids in a long polypeptide chain will be in, as there are only two that will be at the ends.)
You can see from the colouring that what is left of the amino acid in the protein — the amino acid residue — has lost the charges of the α-amino and α-carboxyl groups, which have become parts of peptide bonds. (A molecule of water is removed in a condensation.)
The properties that a particular amino acid endows to a protein are those of the ‘R-group’— R2 in the diagram. In the case of glycine, R is H, so that in the context of proteins we regard glycine as a non-polar amino acid (The C-H bond has a negligible dipole.)
Don’t confuse R with residue. It is a side-chain, designated using a common chemical abbreviation.



The second question was “is the backbone of the macromolecule nonpolar?”.

No. The backbone of a protein has partial charges on the oxygen and nitrogen atoms of the peptide bond, as shown below. This is because it has a partial double-bonded character. These partially charged atoms can (and frequently do) form hydgrogen bonds, either with another part of the backbone (secondary structure such as α-helix or β-sheet) or with polar side-chains.



Reading

Berg et al. Section 3.2 covers the basics (although it fails to emphasize the difference in properties of amino acids and amino acid residues) and Section 3.3 covers the role of the peptide bond in protein secondary structure.