In my experience it is quite normal to close down for training sessions and I think it is great to be able to train the whole team together, work through any issues and have the team able to collaborate with each other. I think those few people who are negative maybe do not fully understand the importance of education.
When we or companies we work with shut down we send out emails to companies, post on our website and social media stating "We are closing for the day to educate ourselves. We want to make our company more experienced for you." This reflects the benefit to your customer. I also think that in the emails / website it is important (also a great conversation starter) to let the customer know what the training will be focused on if it is going to improve the quality of work. It will keep your customers feeling like they are included in the company and if they have been looking at doing something similar, encourage them to ask you questions to confirm if they should implement a similar training day.