RE: Why are Campaigns automatically changing prices before the campaign is selected on the sales order
Happy New Year,
If the Campaign Target check box on the Campaign FastTab of segment card is selected, the sales prices/line discounts are activated for all contacts in the segment. If the sales prices/line discounts are not to be offered to all the contacts in the segment, you can clear the Campaign Target check box on lines for the contacts in question.
Understand that if you uncheck the Campaign Target, the Campaign pricing will not auto populate. You will not see the Campaign under the Sales Line/Line/Function/Get Prices (pic below).

Hope this answer your question. For me I would keep it active, since you are activating the campaign and the customer, buying the item, deserve the campaign price discount.
Thanks,
Steve