Yes, some do register the seedlings they purchase and others just use it to hybridize with.
A lot if times the seedlings are good enough to register, however, it may not fit into the hybridizer's program so they sell it to someone who can use it.
Some people only have a very small space to grow registered cultivars as well as seedlings so there is no way they can keep all of the seedlings that may be worthy of registering, they keep the best of the best so to speak.
Also, some are being used mainly for hybridizing purposes and with limited space once the seedling get so big then it has to be divided and some of it sold.
