Actually, you can decode MPEG2 on a Raspberry Pi without the decryption key. The difference is that without the key, you can only use software decoding, which makes real-time playback of HD content very challenging considering the Raspberry Pi's hardware (I've noted some dropped frames on mine).
As per the Raspberry Pi Website, purchasing the license will
enable a single Raspberry Pi to decode MPEG-2 video in hardware. The Raspberry Pi is not the most powerful device, and with a single core 700 MHz ARM processor, it might have trouble decoding higher resolution videos. In this case, you might want to consider purchasing a decoder key to unlock the hardware decoder (which should be capable of decoding even Blu-ray quality MPEG streams).
Note that if you are using a Raspberry Pi to play back these videos, some Blu-ray rips (playing back Blu-ray movies directly can be challenging on Linux due to licensing requirements) also use the VC1 codec; there is another applicable hardware decoder you can purchase for that as well - should the software decoder not be fast enough for your needs.