If people want choice, other than the large manufacturer’s, then they should do everything they can to support the innovative companies in the space. Buying your supplies from your pump manufacturer, if possible, is a small thing and can help ensure that Tandem is around.
I would guess that the issue is one where without the proprietary connection some of the 3rd party suppliers have been substituting non-Tandem products to increase their bottom line. Having a proprietary connection will hopefully increase their profit and allow them to stay in business. If you haven’t seen their stock price lately it is enlightening.