It doesn't look much different than the protections a retail store currently have. They can detain you until the police arrive and they are not liable for criminal or civil liabilities either (doesn't stop people from trying to sue).
But in this day and age a good thrashing on twitter and facebook usually ensues.
1. It's equally wrong for retail stores to have that protection. No particular class of business should be entitled to more protection than other types of businesses or ordinary citizens seeking to detain someone who committed a crime against them. I also highly doubt retail stores have a federal law giving them this legal protection.
2. This law doesn't just permit theaters to detain you until the cops arrive; they're permitted to detain you for the sole purpose of questioning you. I think it's extraordinarily problematic for private security forces to be given the power to do that. Such a detention could go on indefinitely, leaving it up to a lawsuit to hash out whether it was "reasonable" or not. If they're just detaining you until the cops arrive, at least the length of detention is upper-bounded by the response time of the cops, and there's no opportunity for shady or coercive questioning techniques.
In case anyone is wondering, that's the us federal law. Most States have individual laws that change the punishment.