To "keep someone in line" is to use your influence or power over that person (or group of people) to basically control them, to make them act how you want them to act so that it works to your advantage - in this case it sounds like they're filing complaints so that other companies get investigated (likely due to "competition law"), and don't have a chance to use any unfair tactics, when they likely have no evidence of them doing so. They may well have evidence, though, and it could be a valid complaint - but most of the time it's just "scare tactics" - they're just trying to spook them.