These voters have already cast their vote for who they want to see on the general election ballot by not registering as a Democrat or Republican prior to the cutoff. Any voter can change his or her voter registration more than 45 days before the primary election to support the candidate or candidates they wish.
I know several individuals, to include family members, who have changed party affiliation to vote for a candidate they support.
Laws currently allow minority parties to place their candidates on the primary ballot and allow their party members statewide to decide who should be on the general election ballot to represent their party. The Green Party was successful in this in the 1990s. By getting support during a previous statewide election, they can be added to the next ballot.
Allowing non-party members to determine who their party nominee would be would adversely skew the election outcome. In years where one party has a single candidate, those party voters would be free to attempt to skew the general election by crossing party lines and voting for a candidate less likely to beat their favored candidate in the general election.
Besides, where would the line be drawn? Would we allow voters from Colorado, Arizona, Florida, New York or California to vote on our elected officials? Would we open it up to foreign nationals, cats, dogs, cows, horses, snakes, coyotes and, God forbid, Texans to decide who will govern this state?
Leave the system alone. If a voter feels strongly enough to support a specific candidate for election, campaign for them, donate to their campaign and, lastly, register as a voter in their party to vote during the primary election.