Some web sites limit your membership to one degree or another when you have a trial membership. Generally speaking, the more popular the site, the more likely this is to happen. For example, they won't let you download an entire sequence of videos, perhaps only the first one, unless you have a "full" membership. www.givemepink.com is a good example of this.
You need to read the cancellation terms very carefully when signing up for trial memberships as these sometimes make it almost impossible to cancel before a full month's membership charge kicks in.