It's frustrating and personally, I find it absurd that any company serious about customer service treats email inquiries with contempt. Unfortunately, it's becoming far too common for many suppliers to show this level of ignorance. They do it because they can get away with it due the fact that they have a massive influx of inquiries coming in, so they have no problem ignoring a certain percentage.
I guarantee you, if they didn't have a large number of inquiries to choose from, they would be knocking your door down to get you to sell their products. It's nothing more than complete ignorance, it drives me crazy on a personal and professional level, but it's just the nature of this business.
The only way you are guaranteed of getting service is by phone contact. I'm not aware of any supplier that ignores phone calls the same way as they ignore email inquiries. But you have to ask yourself, would you really want to do business with a supplier that has to attitude to customer service to begin with, especially if you are talking about Drop-shipping?
What happens when you have a customer needing a problem sorted, is the supplier going to just ignore your request for a resolution and leave you holding the can?
For me, I don't think it's worth getting upset if they ignore an email inquiry, I would take that as a red flag against their level of customer service and then move on. they clearly aren't someone that is capable of remotely interested in delivering you a satisfactory level of service to begin with.