Ahem not really, they use the webkit rendering engine, they can then put their user interface, include persistent storage, or do what ever they want to do and build around it...Or simply put they can choose what ever they want to do during the events that are triggered for application close, application suspend, application resume, application forced close, you name it...And not just that, developers have plenty of opportunity on how they want to run and manage their objects.
Sure keeping everything in RAM all the time is quick and instant accessible, however you never have enough. An increase in RAM could help for some as a temporary patch. There is nothing stopping any one to write the current memory state to disc, and upon resume load it from disk opposed to the internet. However don't forget websites also update their data, so which pages do you want to store/cache and which do you want to have reloaded? And what about those that will force a reload anyway when you have 16GB of RAM in your device?
I get that some may find it an annoyance, but think a bit more about the implications and what is already possible.....