BMW offers the CAN-BUS socket charging for a reason...possible control of charger type limited to those with the proper output plug and K1600 compatible. They may assume such a charger is compatible with the K16 electronics. Their's is.
Yes, we can buy an adapter to mate standard battery tender outputs to the CAN-BUS receptacle, but the system won't allow the charger access to the battery for very long. Optimate for example had to reconfigure their Optimate-4, already BMW compatible on other models, to work with the K1600.
I doubt a dealer would be following BMW's advice if they bypass that "feature" with the traditional adapter found on other motorcycles. Owner's info strongly suggests disconnecting the battery before charging if a standard tender is used to protect the electronics.
There's always the chance that someone will hook up a charging system built for old-style flooded batteries with an uncontrolled voltage/current supply and dirty DC output. If something blows there's no warranty if BMW finds out a non-compatible charger was in service.
Gary