My understanding is the same as Ricky's. At least in the Broadcom word, you have to license the SDK from Broadcom in order to develop against it and, more importantly, have documentation of which register does what. I don't know if you need to license it to program the ASIC (assuming you can do it without SDK in a sensible fashion).
If you can live within SAI's capabilities, SAI is freely available for Broadcom. This is what SONiC uses to communicate to the Broadcom chipset, and it works well. If you really want a rundown, please ping me off list; I have a good bit of information I could share from prior presentations... You can also look up my presentations at nanog/lacnog on the topic, but I'm glad to fill in what I know beyond what's there if it's helpful. 😊 Russ