Incremental Circuit Of This Amplifier
This is a small signal equivalent circuit, which is used to analyze circuit behavior for a small ac signal. For that reason, all capacitors and DC sources are treated as shorts.

For instance, the VCC and the emitter are shorted to ground. As a result, in the equivalent circuit, R1 and r&pi; appear in parallel with R2. 

The circuit has a negative feedback resistor, RB, connected between the collector (output) and the base (input). This is an ac feedback (due to CI, it does not affect the DC operating point of the transistor) and, therefore it should be included in the ac analysis.

As suggested in the comments, the equivalent circuit does not show the effect of this feedback, i.e., it is representing an open loop, where the gain and other characteristics of the circuit are analyzed with the feedback broken. Based on that analysis, closed loop characteristics could be calculated later.

Still, when open loop is analyzed, the effect of RB has to be taken into account, since it presents additional load both for the input and for the output signals. In the equivalent circuit, RB is connected directly to ground, in both input and output sections, which does not quite match the circuit - it is likely an approximation, since RB is substantially bigger than RI and RC.