"If you don't want your messages going via iMessage, Settings -> Messages -> iMessage -> Off. Was that so hard?"
If they don't know the phone is doing this (and from the class action suit it would appear many people don't) and is doing this by default then for most non-technical users (and let's be honest that covers 90+% of people with any type of phone) they may wonder why the messages are different colours, but most won't realise this 'feature' exists until they try to change phones.
Honestly - I don't own an iPhone, but my wife does, and the only reason I know that this happens is through news on El Reg. I suspect the majority of (non-techie) users assume that if they respond to a text message in what appears to be a text messaging app it will reply with a text message.
Would be interesting to run a survey to see how many (average/non-reg-reading) iDevice users are aware of this functionality, and more importantly are aware of the pit-falls.