I think if the bank has a physical location then the customer should be able to go and get their money there. If they are an online bank and the website needs to be update then of course it makes sense that customers can't get their money. That's the downside of having on online bank and it makes sense that sometimes they would need to ''close''
Even though must people think of banks as a service that helps them. It is a business that needs to protect itself from loss. If every customer withdrew their entire balance, it would put the bank at great risk. When you enforce a rule, it has to be inclusive, otherwise it has no effect.
No they should not be able to hold any ones money at anytime. Just because they are doing maintenance does not mean we as customers do not need our money. What if someone needs that money at that exact time for an emergency and it is not available. That can be a law suit in my opinion.
When a bank runs online maintenance, customers often can't access their money information online. However, such maintenance shouldn't prevent someone from getting their own information. Some outages outside of maintenance shouldn't prevent someone from accessing their money either. Without a doubt, consumers deserve access to their accounts at all times no matter what.