It's not a regular CharlieCard. It's a special card whose only purpose is to unlock the bicycle cage. That's by design, to limit the number of people who have access to it.

Alewife commuter Dan Dunn remains a bit skeptical that the cages will actually deter theft or that the T will actually keep the cages maintained:

Maintenance. The T simply sucks at it. They don't have anything that doesn't break down. Trains, turnstiles, parking lot gates, ticket dispensers, signals, tracks, bridges, speakers, coin dispensers, buses, dollar-bill readers, pavement, batteries, rechargers, refuellers, radios, electronic signage, locks, security systems, etc. If the T has it, it breaks. Let's see if this cage is still intact and functional in 6 months or 6 years.