It's not so simple. First of all, if a sideloaded app causes a security breach, Apple will receive blame, possibly all of it. Second, these apps may not obey the rules of iOS when it comes to security, and if they open access then it's possible a hacker could ride that into the network the iOS device is on...again, Apple would receive blame. Basically all of the consequences of crappy apps would fall onto Apple, from security breaches to resource hogging to taking our information without notifying us. And that's without getting into the malware that you might pick up from apps or a web browser that allows things to run on the device.
Apple does not have a monopoly on devices in this class, whether you're talking about phones, tables, or stores...there are many, many options out there. Apple has chosen to set up their devices to work a certain way, just like others do...the real difference is that Apple has created a way to keep their devices more secure, and the users appreciate that difference. The dues for developers were created in line with the rest of the industry, and no one is suing to drop the prices at other stores. This is really about Epic trying to game the system so they can have all of the profits without paying for anything...it's just like rich people not paying taxes but driving down the road and having the police and fire department protect their homes.
Apple does plenty of things that are annoying, but in this case they are in the right, it is their ecosystem, they developed the rules, and others need to play by those rules. Everyone is treated equally, it's just some companies who want all of the advantages without any responsibility towards supporting the platform that are at issue.