QBP has a giant catalog. That's the first distributor account most bike shops open. For applications with other distributors, they often just want to know if you have an account with QBP. If so, they'll give you an account as they figure the vetting QBP did was good enough for them. All of the brands you mentioned, Dan, are in the QBP catalog, but may be limited to just a few products. Not many Nitto products, for example.
What has not been mentioned yet in this thread is that for pretty much any distributor account, you're required to have a resale certificate for sales tax purposes.