Why exactly is circumventing region locks a bad thing?
First off I didn't say it was bad, however from the media companies perspective I can see it as bad. From the audiences perspective it's good, at least in the short term.
why is the customer not allowed to use the global market too?
You are not a customer, you are a product that the network also sell to advertisers.
Most goods are not regionally licensed. It's bad for media companies, for now, because the region locks are there to protect revenues of the content creator. Current licencing is by region. That is a flawed model, but they are very tied into it. Many existing contracts between distributor and broadcaster, as well as some laws, are based around regional licencing.
There are real costs associated with creating content. Currently most of those costs are recovered by targeted advertising. By avoiding region locks, you're more likely to get irrelevant ads. That's wasted spend for the advertisers who will see their ratio of conversions fall. (A viewer who acts on an ad.) They will thereby reduce advertising by either negotiating a lower price per viewer, or buying less ad space. Under the current system, it takes time to line up shows with advertisers and audiences. In some cases expensive shows are broadcast to line up with other regions seasons. E.g. In summer shows do not do well as most people are outdoors. Shows produced in the northern hemisphere's winter are delayed 6 months to line up with the southern hemisphere's winter. This ensures a high number of viewers, which will likely lead to a higher conversion rate for advertisers. If the audience have already seen the content by avoiding region locks, there will be lower viewer numbers, advertisers will not spend as much, and the networks will look for less expensive content. The content producer sees less profit, and take fewer risks creating content.
In short, the media companies are tied up, and slow moving to keep pace with how fast content can now be delivered globally. The current distribution model is flawed as it was created in a world without the internet. It is written into laws and contracts that are hard to change. Circumventing the current model is bad for the media companies in the short term and bad for audiences in the long run as shows will have lower budgets (think reality tv). However it's great for the audiences in the short term as they get to see content whenever and wherever they like. Media companies need find a way for the audiences to enjoy the content delivered immediately to audiences who want it wherever they are, while maintaining their revenue and profits. This will require a large effort, with changes to regional laws, trade agreements, licencing contracts etc.
In my opinion the best way to avoid all of this is for content producers to sell directly to their audiences. This is hard because good content producers have high costs, and high risk. For every great show you see on TV or great movie you see at the cinema, far more were produced and failed. That cost is also currently paid for by advertisers and out of the profits of successful shows. Things like kickstarter might start to allow for those risks to be taken, but even producing a pilot to start a kickstarter is expensive. Some of the profit from successful shows needs to go into all the failures that will occur before another success. This is why we see crappy how to videos on youtube and not high production value dramas. It's also why the current media companies are the only ones to produce that type of content.
Shows are produced by production companies. That product is sold to networks. The networks sell the show to advertisers. The network is the customer of a show and the advertiser is a customer of the network, not you the audience, you are not a customer, you are a product that the network also sell to advertisers. The real customers of shows, networks and advertisers do use the global market.