In a way they already do, David.
I was boarding a NWA flight last week when a woman ran up the the Gate Agent and demanded to know why I had two carry-ons while she was allowed only one. The agent (who was pretty sly) merely smiled and said, "We usually only allow on carry on, but if you would like to buy a 1st class ticket too, we'll be happy to let you have two carry-ons."