the program does not discriminate against any candidate or point of view, and it does not restrict any person's ability to speak. In fact, by providing resources to many candidates, the program creates more speech and thereby broadens public debate. ...(See my previous argument against the Institute for Justice's position on this, with some subsequent clarifications on other aspects of the law.)
At every turn, the majority tries to convey the impression that Arizona's matching fund statute is of a piece with laws prohibiting electoral speech. The majority invokes the language of "limits," "bar[s]," and "restraints." ... It equates the law to a "restrictio[n] on the amount of money a person or group can spend on political communication during a campaign." ...
There is just one problem. Arizona's matching funds provision does not restrict, but instead subsidizes, speech. The law "impose[s] no ceiling on [speech] and do[es] not prevent anyone from speaking." ... The statute does not tell candidates or their supporters how much money they can spend to convey their message, when they can spend it, or what they can spend it on. ...
In the usual First Amendment subsidy case, a person complains that the government declined to finance his speech, while financing someone else's; we must then decide whether the government differentiated between these speakers on a prohibited basis--because it preferred one speaker's ideas to another's. ... But the speakers bringing this case do not make that claim--because they were never denied a subsidy. ... Petitioners have refused that assistance. So they are making a novel argument: that Arizona violated their First Amendment rights by disbursing funds to other speakers even though they could have received (but chose to spurn) the same financial assistance. Some people might call that chutzpah.
Indeed, what petitioners demand is essentially a right to quash others' speech through the prohibition of a (universally available) subsidy program. Petitioners are able to convey their ideas without public financing--and they would prefer the field to themselves, so that they can speak free from response. To attain that goal, they ask this court to prevent Arizona from funding electoral speech--even though that assistance is offered to every state candidate, on the same (entirely unobjectionable) basis. And this court gladly obliges.
The majority position on this issue is that the unconstitutionality arises from the way that the subsidy to clean elections candidates is tied to campaign spending by the non-clean-elections candidates; I take it that had the subsidy been a fixed amount the argument would not have worked at all.
There's a good overview of the issues at the SCOTUS blog.