Because hundreds of thousands refilled or remanufactured cartridges are printing daily in all corners of the world. In other words, the law requiring to use original only cartridges is honored more in the breach than in the observance. A remanufactured cartridge is a cartridge refilled via an industrial process and delivered to perform as new. If test is passed, cartridge is cleaned, flushed through a flushing machine and readied for refilling. Cartridge is refilled, tested for proper operation and weighed.
Weight is a measure that the proper volume of ink has been injected.
Cartridge is packed for shipping. User is NOT obliged to utilize supplies furnished by the original manufacturer, to maintain the warranty. There are limits here too if the manufacturer can prove that the supplies do not comply with the standards required by the original equipment then he can refuse to honor the warranty. But this is impossible. The supplies, in this case, are the inks and the inks are guaranteed to match or exceed the required specifications.