Hi Nicole and welcome to the forum,
It's certainly not ideal when suppliers make the decision to sell direct through open avenues, but it's becoming quite a common marketing technique for many suppliers to do so, at least the smaller dropshippers. To them a customer is a customer, they don't draw a line between trade and public, they just see it as a viable sales avenue to increase their sales.
Not ideal for someone wanting to develop a dropshipping business model, but not impossible. Of course many will prefer to deal with a supplier who isn't selling direct, but if they are then there are a couple of things you need to clarify.
1. are they selling on the same platform as you?
2. are they selling at the same price as they are offering you?
If the answer is yes to both of these questions, then your options are really limited if your intention is to compete against them. You can perhaps try bundling if you really want to deal with them, see details on that marketing technique on this link Link hidden: Login to view