That's the neat part: you don't have to, because copyright was never a property right to begin with.

First, not only are ideas not property, they're pretty much exactly the opposite of it. I'll let Thomas Jefferson himself explain this one:

It has been pretended by some (and in England especially) that inventors have a natural and exclusive right to their inventions; & not merely for their own lives, but inheritable to their heirs. but while it is a moot question whether the origin of any kind of property is derived from nature at all, it would be singular to admit a natural, and even an hereditary right to inventions. it is agreed by those who have seriously considered the subject, that no individual has, of natural right, a separate property in an acre of land, for instance. by an universal law indeed, whatever, whether fixed or moveable, belongs to all men equally and in common, is the property, for the moment, of him who occupies it; but when he relinquishes the occupation the property goes with it. stable ownership is the gift of social law, and is given late in the progress of society. it would be curious then if an idea, the fugitive fermentation of an individual brain, could, of natural right, be claimed in exclusive and stable property. if nature has made any one thing less susceptible, than all others, of exclusive property, it is the action of the thinking power called an Idea; which an individual may exclusively possess as long as he keeps it to himself; but the moment it is divulged, it forces itself into the possession of every one, and the reciever cannot dispossess himself of it. it’s peculiar character too is that no one possesses the less, because every other possesses the whole of it. he who recieves an idea from me, recieves instruction himself, without lessening mine; as he who lights his taper at mine, recieves light without darkening me.

Second, a copyright isn't a right, either; it's a privilege. Consider the Copyright Clause: it is one of the enumerated powers of Congress, giving Congress the authority to issue temporary monopolies to creators, for the sole and express purpose "to promote the progress of science and the useful arts." Note that that's a power, not an obligation, and the purpose is not "because the creator is entitled to it" or anything similar to that.

Besides, think of it this way: if copyright were actually a property right, the fact that it expires would be unconstitutional under the Takings Clause of the Fifth Amendment. But it does expire, so it clearly isn't a property right.