It's hardly about a company having a "right" to stop you from saying anything, it's about what your reaction to life events says about you.
If you get fired and your reaction is to go to your blog and whine about losing email access and not being able to deliver a talk that you had prepared for x time I, as a prospective employer, am going to draw some conclusions about your personality that won't help you get the job I'm offering.
If you get fired and your reaction is to go to your blog and whine about losing email access and not being able to deliver a talk that you had prepared for x time I, as a prospective employer, am going to draw some conclusions about your personality that won't help you get the job I'm offering.