I think the default behavior of NAV software changed at some point because now, it resets the top speed on each journey it seems(my Nav 4 & 5 kept the top speed no matter what but my 6 is reset each time the bike is not used for a prolonged period).
Top recorded speed probably can't be used on it's own to charge you (especially because it sometimes freaks out and post completely bollocks figures close to 1000 Km/h if you cross time zones and stuff). But in case of accident, the trace recorded in your GPS if left alone could really snitch on you... Not sure it can be used as evidence per se, but it can paint a very unflattering picture of you and make things worse if you have a brush with the law.