Yes, it'd be violating what's known as antitrust laws---laws designed to prevent monopolization of industries and keep *some* competition present.
Disney, Time-Warner and Viacom merging? Doubt the FCC/federal govt. would approve such a thing (even in their most free-market-frenzied state)...
As it is IMO, media is already too concentrated into too few owners *now* (such vertical integration being one reason you don't see Warner Bros.-made stuff on networks they don't own, or Disney stuff on networks *they* don't own, etc.)... having even fewer companies controlling everything would be disastrous, and not just for watching cartoons...
