I believe they did that because of the abuse that was happening with out-of-staters with high registration and/or sales taxes registering vehicles at a mail service while maintaining a domicile elsewhere. I doubt that it would stand up against a serious legal challenge by someone like a FullTimer, but I'm certainly not going to spend the legal fees to find out.
Here's a quote from the 2004 Oregon Voter's Guide regarding the homeless and voting in Oregon:
Note: Registration of Homeless Persons
It is prohibited to deny any qualified person who is homeless the opportunity to register
to vote. For voter registration purposes, the residence of a homeless person shall be any place
within the county that describes the physical location of the person. The mailing address of a
homeless person may be the County Clerk’s office. [§ 247.038]
So where is the line drawn between persons who change physical location annually, monthly or daily with regard to some things that are deemed to be almost rights, like driving...
BTW, notice that even the homeless need a mailing address to vote

ON EDIT: I forgot to mention that last time I checked, Florida law allows DL to reflect the address of a friend!