Except that the arresting officer himself described the knife as a “spring assisted, one hand operated knife” in his report. That makes it pretty clear that the knife is/was not a "switchblade".
Spring assisted is the key phrase.
Spring assisted knives are legal under both Maryland law and Baltimore City Code.
I think it depends on how the spring operates. If all it takes is pushing a button and then the spring does the rest, it probably is illegal. If it is one of the spring assisted knives that you start opening with your hand, then it MAY be legal...depends on the wording and how courts have interpreted it.
http://www.mdshooters.com/showthread.php?t=96897
“The law defines a “switchblade” as any knife having a blade that opens automatically by hand pressure applied to a button or other device in the handle, or by operation of inertia or gravity.”
In any case, for a legal arrest, all that is needed is a reasonable belief that X was illegal, not proof. That is a pretty low standard.