Adam is losing the use of his basement for the month. He could otherwise have stored his things down there, or had a sleepover party down there, or fermented wine down there, or set up a Bitcoin mining cluster. The fact that he can't do these things for a month represents a cost to him that he recuperates in his rent.
It's unfair that Bob should pay Adam for work that Adam isn't doing. Adam can enjoy the benefits of doing those things if and when he does them.
But he can't do them
because of Bob, which is why it is fair to expect Bob to compensate Adam for the loss Bob is causing.
If Adam breaks Bob's leg, and this causes Bob to be out of work for a month, would it be fair or unfair for Adam to pay Bob his month's wages as compensation for the loss Adam caused Bob to take?