Blasphemy is defined by thefreedictionary.com as: "The malicious or wanton reproach of God, either written or oral," and while this is no longer a crime in Western nations, it most definitely is a crime in some present day countries that do not broach disrespect of their God and treat blasphemy essentially as apostasy to their State religion and will enforce those that blaspheme, to such an extent, that those offenders' may actually meet State-sanctioned death.
When you think about blasphemy, you might well ask the question, as to whom this crime is actually hurting, since in the scheme of things, you would think that God being omniscience, omnipotent and so forth, probably isn't taking our deprecations and cursing no matter how vociferously expressed, personally. This leads to the thought that if God isn't striking us dead or mute, why should the government, or its representatives have laws against blasphemy in the first place?
The answer lies more in the fact that within our jurisprudence system, it is common practice, for those giving testimony, to swear an oath to testify to the truth, the whole truth and nothing but the truth, so help them God. While there are some that choose instead to "affirm" to the truth and subsequently are under pains and penalties of perjury, in either case, though, the point is held to be crystal clear, that a court of law is about the whole truth and woe unto him who would fail to recognize the solemnity of such a vow. This means, that those that have a somewhat cavalier attitude in regards to their own swearing, blaspheming, or taking of our Lord's name in vain, are probably also the type of people, that have a tendency to bend truth as opposed to testify to it.
Additionally, history is replete with governments, typically monarchies, but just about any government consisting of this same basic footprint; to which there is the supreme leader, such as the King, who is God's representative here on earth, and that the power that the King has is power granted to the King by God. This than signifies that those that disrespect God are in point of fact, disrespecting the King, and the thing is, the King is a living, physical being, and most Kings demand respect as well as typically desiring to be feared by their subjects, so therefore they often feel that if they allow their populace to openly express disrespect, that it will only be a matter of time before absolute chaos ensues, so that is why there are such stringent laws against blasphemy on record in most countries of any note.
While a country such as America, has no King, it nevertheless has historically through its laws been against blasphemers because a man that is no respecter of God, is probably also someone that doesn't have any particular respect for authority in general, as well as most likely being a man that believes he is himself the best judge of what is right and what is wrong, and that type of loose cannon is not the type of citizen that any structured government wants to deal with.