Not copyright, but trade secret misappropriation.
The Uniform Trade Secrets Act is where to start. In California, its codified under Section 3426 to 3426.11 of the California Civil Code. What may trip up Blizz in this case is Misappropriation - "Acquisition of a trade secret of another by a person who knows or has reason to know that the trade secret was acquired by improper means." And that includes "Derived from or through a person who owed a duty to the person seeking relief to maintain its secrecy or limit its use". The UTSA expressly defines "improper means" to include "theft, bribery, misrepresentation, breach or inducement of a breach of a duty to maintain secrecy". The plain language of the statute and the Comments emphasize that both reverse engineering and independent development are not misappropriation, which means Blizz (and interestingly enough, the bot authors) are free to reverse engineer each other operationally - but trying to acquire the source code is probably breaking the law, at least on the face of things. The law plainly says that anyone who obtains by subterfuge or outright taking any information he has reason to know is confidential, is guilty of misappropriation. If a person obtains information directly or indirectly from someone who does not have authority to disseminate it, that person (or corporation in this case) may be liable for misappropriation by wrongful acquisition. The fact that the person under an obligation of nondisclosure (the bot emplouee or subcontractor) willingly or accidentally disclosed the information (source code) does not protect the recipient (Blizz). Thats the law.
This paints Blizz in a pretty bad legal light *if* whats is alleged (attempts to buy or coerce source code containing trade secret information, i.e. the the protected information of the bot company whcih was derived from the legal reverse engineering of the Blizz functionality).
Where this might bite Blizz is the penalties set forth in the law: At a minimum the bot company/author may recover attorneysâ(TM) fees for bad faith tactics in trade secret litigation or willful and malicious misappropriation (Cal. Civ. Code  3426.4), recover compensatory damages for loss (Cal. Civ. Code  3426.3(a)), and this is the kicker: they have the right for recovery of "exemplary" (punitive) damage award if the misappropriation is "willful and malicious" (Cal. Civ. Code  3426.3(c)).
On top of that, the California Business and Professions Code  17200 prohibits "any unlawful, unfair or fraudulent business act or practice." This statute is extremely broad in its coverage of prohibited acts and practices, and it creates a private right of action for redress of any practice forbidden by any other law (civil, criminal, federal, state, municipal, statutory, regulatory or court made). So further sources of actions may be contained there - but thats so broad that I'm not going to speculate on what creative use of that law the parties may use against each other.
So that's the civil side. Here's where the real fun comes in: Criminal code. As in someone might become a felon, and/or go to state/federal prison. Federal and California laws address this - California first: Cal. Penal Code  499c "Theft of Trade Secrets". It subjects trade secret misappropriators (and persons conspiring with them) to criminal penalties if they appropriate trade secrets by wrongful or dishonest means, or if they offer anyone a bribe to do so. [emphasis added] . A violation is punishable by a fine of up to $5,000.00, imprisonment of up to one year, or both. The violation can be either a misdemeanor or a felony, depending on the value of the trade secret stolen.