Alfredauto is correct. Anybody with tools can remove/install a hybrid battery, but any shop that goes into this without safety training and equipment is facing a major liability. Keep in mind these are lethal voltages we are talking about. When you turn off the master switch, the car is supposed to self-discharge - but does it? If not, those lethal voltages are still there.
With THS you get that training and more. Training covers control systems, inverters, DC to DC converters as well as drivetrain and battery theory. A week of theory and practical for your techs plus you get to send a tech to SAE cert training. Then you have ongoing access to expert support people.
As regards aftermarket batteries, I challenge any aftermarket supplier to let me run one of their off-the-shelf units through the THS testing process to document whether it is in balance and capable of supplying rated power and energy. Every battery I condition comes with documentation to show that it is performing at 95% of new specs. Also, these batteries discharge while sitting in storage. The battery should be charged to 60% of full capacity just before installation.
It's analogous to engine replacement. An aftermarket battery is like a used engine from a junkyard you trust. A THS battery is like a remanufactured engine from a reputable machine shop/supplier. Each cell pack has been tested for total performance (not just voltage) just prior to installation. That performance is documented and the warranty is backed by a nationwide chain of THS shops. The THS process and equipment let you control the quality of the product you are installing.
There is a lot of discussion on this forum about providing value for our customers. This is a classic example.