Why?
The exception to the law (no non-Catholic service in Catholic houses of worship) was made in favor of other Christians who did not have access to facilities large enough for their services.
"Catholic churches are consecrated or blessed buildings which have an important theological and liturgical significance for the Catholic community. They are therefore generally reserved for Catholic worship. However, if priests, ministers or communities not in full communion with the Catholic Church do not have a place or the liturgical objects necessary for celebrating worthily their religious ceremonies, the diocesan Bishop may allow them the use of a church or a Catholic building and also lend them what may be necessary for their services. Under similar circumstances permission may be given to them for interment or for the celebration of services at Catholic cemeteries." (Directory for the Application of Principles and Norms on Ecumenism, 137, 25 March 1993)
There is a facility available for this funeral that is much larger than the Washignton Cathedral. The only reason our Church is being loaned out to heretics is because the dates weren't convenient for the family to use a Protestant building. This sort of circumstance is certainly not what was envisioned in making this exception, since the exception made to the law is in favor of the ministers and community of the sects, not the faithful of the sects for private acts (a funeral), and is an exception revolving around a lack of place, not an inopportunity of time.
However, on a bigger question, I find the application of this exception is frequently at least gravely scandalous - i.e. lending our Cathedrals to Protestants to "ordain" Clericettes. The whole procedure is anti-traditional and would be better in my view if it was stopped.
Making an exception for Rhenquist here is simply opening up the exception to the law a little further in the continual effort of AmChurch to open wide the doors to heresy.