Why is non-inverting op-amp input resistance on a tee to ground?
Let the resistance looking into the non-inverting input be \$R_{in+}\$

If a resistor \$R_i\$ is placed in series with the non-inverting input, the resistance seen by the source \$v_I\$ is

$$R_I = R_i + R_{in+} \ge R_{in+}$$

If a resistor \$R_i\$ is placed in parallel with the non-inverting input, the resistance seen by the source \$v_I\$ is

$$R_I = R_i||R_{in+} \le R_{in+}$$

Since, for an ideal op-amp, \$R_{in+} = \infty\$, the series case gives

$$R_I = R_i + \infty = \infty$$

while for the parallel case

$$R_I = R_i||\infty = R_i$$