I'm just wondering, when you were deleting a lead from Marketo, and it asks you if you want to "Remove from SFDC", wouldn't it be better if that defaulted to "False" instead of "True"?
I had an example where someone imported a list of contacts into Marketo and realized that they were the wrong contacts for that list and didn't want them in Marketo, however they lived in SFDC and they wanted them to stay there.
In their haste, the opted to delete the leads from Marketo, and without thinking, clicked "Run" while "Remove from SFDC" was set to "True".
In as situation like this, it would be great if the Adminstrator has the power to set the default value of this operation to "False" to avoid anyone from accidentally deleting leads from Salesforce.
You'd probably be better off suggesting this as an idea: https://community.marketo.com/MarketoIdeaNew
In general, I get what you're saying, but I think you could argue it either way. What might be a better solution, is to set it to something neutral (e.g. "---") and requiring a selection before you can continue. That way no accidents can happen, but you're also not assuming one action over another.
I agree with Matt that you should post this as an idea. My guess would be is that Mkto sees itself as a parent database where all information is stored. Especially since you can control the flow of information from Mkto to SFDC. I think they make the assumption that if you are deleting from Mkto, then you must be deleting completely.
Just my impression of their thinking. But definitely create an idea!