TRAI rules give them this freedom to do so, If you don't have balance your sim will be deactivated and on that you can't do top-up recharge without active plan.
True ARPU greedy is Jio not Airtel. Lower income people has cheaper option before Jio's monopoly in the 4G market. A Rs. 10 recharge could last months. Now we have to spend over Rs. 100 on even state owned operator like BSNL just to make calls. Nobody wants unlimited calls. People would be happy with Rs. 50 talktime. Guess what you don't have any options here? Unless you opted for Jio's carrier locked feature phone. This whole unlimited thing is to tie you with operator for the lifetime.