You don't sound bitchy, it's just a decision that we each make separately as bookwrym said.
For myself, I'm kind of with you. There are a fair number of really good corsetmakers out there these days who all have different interests, so there's no shortage of people ready and willing to take a commission that you aren't... If you're fortunate enough to be in a position where you can focus on your interests and turn down orders that don't capture your imagination then why wouldn't you? Yes, there's nothing to stop someone doing whatever they like with your work once it's theirs (not just alterations, but breaking it apart to look at the construction and all sorts), but that doesn't mean that you have to like or invite it
So for me, I don't take commissions on plain pieces for someone to embellish after the fact (except on rare occasions when it's just the addition of crystals and I trust that person's abilities). If they want a very plain corset for that purpose then there are other brands they'd be better suited to working with. I want to create things for people who chose to work with me *for* my aesthetic, not despite it. It isn't about being bitchy or snobby, it's about recognising where you want to be within the market and sticking to that goal.
One-woman businesses (especially artistic ones) do not have to be all things to all people. If we want to specialise and restrict the type of work we put our name to, we can. Whats more, I personally think the industry as a whole ends up more creative and interesting when we do.