You think something that makes a business more profitable in one country wouldn't be done in another if it was permitted? All regulatory bodies pass bad regulations from time to time, usually because of unintended consequences but having universal agreement across the EU almost always makes the regulation stronger as it is reviewed and agreed by more people.
It may be that it takes slightly longer to react to change with this process I suppose, but then at least the right decision is usually eventually made. This is often not the case in somewhere like the US where lobby groups and politicians can hold things up for years or even block them completely if big business wants them to. The European consumer protection legislation is infinitely better than most of its American counterpart.
Then of course there is the really obvious benefit of having only one lot of civil servants to pay, not 28, and only one set of regulations to follow, not 28.
It depends mate. Look at Switzerland. You still might have all the benefits of the free trade, free moving of people and so forth even adopt some laws but have your own legislature.
You don't have universal solutions because each local market is different. Some cost of goods or manufacturers are not competitive within the local market simply because the cost of production is higher when you take into consideration the labor force and the standard of living. So it's tough to draw the line especially if you try to enforce universal laws and regulations.
Sometimes as a local manufacturer you pass regulations that you have already fulfilled by local regulators with different name. Sure it is fixed after some time and generally mistakes are made and fixed, but during that time it's a disadvantage for locals who already have high quality products and goods.
No system is perfect, I can understand where some are going when it comes to fulfilling EU regulations and generally most of them are not a bad thing, but still that is valid point in some case so many voters that have this personal interest is perfectly normal to go behind that campaign.
The problem is - most that do vote for Leave are not educated to the effects of the vote, don't really care, or are mislead by unrealistic expectations.
However it is still democratic if you are an idiot to vote, so unless the referendum is rigged you really can't do anything about it.