Because I'm a goddamn citizen
Some friends have been deriding the recent ruling that ordered a local cake-maker to serve gay couples, saying it is against the principles of the country. It is anti-freedom, or anti-liberty, or something similar. These are fairly liberal friends. I believe they are wrong.
In the legal sense, this falls in line with the Federal Civil Rights Act. It guarantees all people the right to "full and equal enjoyment of the goods, services, facilities, privileges, advantages, and accommodations of any place of public accommodation, without discrimination or segregation on the ground of race, color, religion, or national origin." And now society is expanding that to include sexual orientation. If you want to discriminate you get to make your business a private members-only affair.
One of the main functions of government is to protect its citizen’s rights. Part of the social contract as most people understand it is the right to walk into a public business and exchange your money for whatever they're selling without discrimination (within reason). You have the arbitrary and unlimited right to not deal with anyone you don't want to on a personal or private basis. But if you're running a public-facing business, you get to treat all Americans equally.
For random Joe Baker to refuse to serve an orderly customer is for him to announce that he is above the government. He refuses to recognize whom the government has designated as a citizen. He will, using his own metric, decide who he deems worthy of citizenship regardless of what the government has decided.
This is a challenge to government power which the government cannot let pass. The government demands a lot from its citizens – taxes, military service, adherence to laws – and in exchange it provides certain benefits. A major one being the assurance that all citizens will be treated equally. If someone has made the sacrifices and met the conditions that citizenship requires, and then the government fails to enforce these rights, there will be much less incentive to be an American citizen. The title must be worth something, and this random schmuck is trying to degrade its value. No strong government can let this sort of challenge go if it wishes to keep people interested in being a part of it.