MultiPlus-II 12/3000/120-50 2x120V recommended CB on AC out 1 is 75A for a 3,000W output?

The user manual recommends a 75A CB on the AC1 output. Is that because it combines the shore-cord power of 50A and the 25A inverter output?

Yes, if you are using power assist be sure the rest of the circuit can handle the load and protect accordingly