For the record - TREC does not regulate commissions, and there is no such thing as a standard commission rate. You can argue a market rate, but there is not a standard rate in our industry.
There is a contract between a Listing Broker and a client which sets an agreed upon commission to be paid to the Broker, and an amount to be offered by the Broker to any Buyers Agents within the formal listing agreement. This can be conditioned on whether they are or are not participants in the local MLS. As many have mentioned, these variables are all 100% negotiable.
The Local Board of Realtors and MLS will regulate any commission disputes between Realtors specifically regarding the situation provided prior in this thread where the Buyer's Agent writes in a larger % than is being offered through the MLS system. That would constiture a breach of the MLS rules, and that agent could be subject to loss of membership from the MLS.