The United States constitution is the foundational document outlining the powers that the federal government possesses, and more importantly, the rights that it does not have. The first 10 amendments, commonly called the Bill of Rights, outline several important rights that the government is specifically prohibited from taking from the people. How the constitution applies to business is for the courts to decide ultimately, and many opinions exist on this subject.
The commerce clause is found in Article 1, section 8, clause 3 of the U.S. constitution. This provides congress with the right and ability to regulate international trade, as well as trade between the states. This power was given to the central government to bring uniformity to trade between the states. Any business that buys or sells items in states other than where it is located, or in a foreign country, is subject to any laws that the federal government may pass to regulate these transactions. The state government retains the right to regulate trade within its borders.
A person's right to free speech is preserved by the first amendment, which restricts congress from making laws restricting this right. Of course, the right can be modified as necessary for the public good and safety. Free speech also applies to a business. This was challenged with campaign finance reform, which restricted a business's right to donate to political campaigns. Some businesses owners objected, saying that it was part of free speech to give money to a political candidate of their choice. On January 21, 2010, the U.S. supreme court ruled in a five to four decision that corporations have the same right to free speech as individuals, and lifted the restrictions on contributions.
The constitution protects the right of free association as part of the first amendment. A person is allowed to form his own belief or association with people, essentially of his own free will. This applies to businesses, as well. Most notably, you do not have the right to hand out information about a personal cause at a place of business without the business owner's consent. He does not need to allow you to associate that belief with his business. This right of free association is limited in matters of employment, as equal opportunity employment defines several instances where employers cannot discriminate in their hiring practices.
The constitution regulates and restricts the powers of congress, and not individuals or businesses. An example of this would be a person demanding that he has the right to free speech, and, therefore, should be allowed to speak on a talk radio program. The constitution does not state that the host cannot restrict free speech, it states that congress cannot. Therefore, the host is free to deny the person their "right" to free speech. Businesses generally have the same rights under the constitution as individuals, but all constitutional law issues are subject to the final decision of the supreme court.