Example of someone has an idea and they tell me and I go and take that idea to market before them they would say I stole it even though they still have their idea.

if that “idea” was patented or trademarked, or if the original creator can prove “first/prior use” in court, theyw ould have a viable civil case for IP infringement under the DMCA (in the US, or under similar laws in other jurisdictions). again, however, this is not “theft” by any legal definition, but a form of copyright infringement as defined by the law. Nothing was taken or stolen (the creator still possesses the original), but, rather, copied and then illegally profited from.

How do you figure that it’s not stealing?

I did not make this determination. different forms of theft are defined by laws:

theft
n. the generic term for all crimes in which a person intentionally and fraudulently takes personal property of another without permission or consent and with the intent to convert it to the taker’s use (including potential sale). In many states, if the value of the property taken is low (for example, less than $500) the crime is “petit theft,” but it is “grand theft” for larger amounts, designated misdemeanor or felony, respectively. Theft is synonymous with “larceny.” Although robbery (taking by force), burglary (taken by entering unlawfully) and embezzlement (stealing from an employer) are all commonly thought of as theft, they are distinguished by the means and methods used and are separately designated as those types of crimes in criminal charges and statutory punishments. (source: law.com)