Some climbing centers and organisations mandate helmet usage even indoors for younger or novice participants.
Safety in Outdoor Education Policy for Schools & Youth Groups1 for the Isle of Man states1:
All participants on outdoor crags should wear helmets and it is recommended that those leading on artificial inside walls also wear them.
That policy was listed in the 2008 policy document.
The UK's The Scout Association POR (Policy, Organisation and Rules)2 state:
Rule 9.34 Climbing and Abseiling
a. Climbing helmets must always be worn by all those climbing or abseiling on natural features, except in the case of 9.34c.
b. Climbing helmets need not be worn by those climbing or abseiling on artificial walls provided the activity leader is satisfied that the climber or abseiler has sufficient skill not to react unpredictably. Novices must always wear helmets, except in the case of 9.34c. The use of helmets for climbing using auto belay systems must be determined by the activity risk assessment.
c. A Sikh wearing a Turban may choose to climb or abseil on natural features and artificial climbing walls without a helmet.
This rule was in the POR from at least 2011.
The Boys Scouts of America appear to have a similar rule3
When not to wear a helmet — Auto Belays
There is a demonstrable risk to climbers wearing a helmet while using an auto belay as there is the possibility of straps getting caught on a hold as they descend.
Adventure Activities Licensing Authority [AALA] Board Meeting; 8th October 20134
July 2013 - A 9 year old girl was descending a climbing wall in a shopping centre when her helmet got caught on one of the holds.
An earlier AALA report from 20115 details a similar incident:
A 6-year-old girl nearly asphyxiated whilst on an indoor climbing wall. She was wearing a harness and helmet and was attached to an inertia wire. Having lost her foot and hand holds, she twisted on the wire and the plastic adjustment strap at the back of her helmet caught on one of the projections, suspending her weight by the chin strap. It took staff members and members of the public several minutes to climb up and release her, by which time she was blue.
The report concludes with:
It may be better to discontinue using helmets on climbing walls and towers.