With Lollipop they are handling disk partitioning differently. Bloat on your typical Android phone is installed on the system partition, because the phone is pre-branded to a carrier, as part of the core OS and can't be removed as a result. With the Nexus 6 the phone is open to all carriers and the bloat will be downloaded based on the carrier network you're connected to, hence the ties to the SIM, but it will be installed to the data partition and is removable. There are no apps on the SIM itself, however.
EDIT: While this is a feature of Lollipop and isn't exclusive to the Nexus, it doesn't mean things will change for all devices. If the carrier molests the OS before sale due to it being locked to their network they are still able to install bloat to the system partition and have it be non removable. The Nexus 6 isn't carrier locked and is the same device for all networks which is the key difference. Carriers should not be modifying anything pre-sale and every piece of bloat should be installed on the data partition as a download based on the SIM you're using and thus be removable.
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