The default behavior in Salesforce is for imported prospects to be assigned to the user who is doing the importing. SalesLoft does not change this.
If you find your leads are being assigned to another user in Salesforce, it may be due to something called a Lead Assignment Rule which assigns incoming leads to other users based on certain criteria. This rule is set in your company's Salesforce instance, and will need to be changed by your Salesforce administrator.
You can read more about how Salesforce handles Lead Assignment Rules here.