Do I need to pay royalties to the SD card people if my device "just happens" to be compatible with SD cards?
I had the same concern just a few days ago.  I am planning on using a microSD card internally inside a device for extra storage.  I will only be accessing it via the SPI interface and not using any proprietary features.  The end user will not even be aware there is a microSD card buried inside the device, so there is no need (or desire) to put a SD logo on the product.

I sent an inquiry to helpdesk@sd.org.  Their reply was "If your client's product will be interoperable with SD cards, they will need to sign the Host Ancillary Product License Agreement.  SD Association membership is required in order to sign the HALA."

So it looks like we have to pay the piper to be completely legal.  My understanding it is $3000 a year ($2000 for the membership, and $1000 for the HALA).