You're not obligated to do that. The difference here may be that there's already a contract between a user and the company, so if they include something like "

You're not obligated to do that. The difference here may be that there's already a contract between a user and the company, so if they include something like "continuing to provide service to my account constitutes acceptance of these terms." in the updated contract, it might fly. Or not. It could be that all the businesses doing that to their users are just BS'ing something unenforceable.