It is not against the law, so no one can really stop you. But this is not the point of a deferral. If you request the deferral, you are making a good-faith agreement that you do want to attend the school but are not able to attend in August 2014. You would be abusing the deferral if you also tried to use that time to try to get into other schools. I don't think people will be very happy with you if you asked for a deferral, then decided that you will apply somewhere else. It's one thing if in August 2015, still more reasons pop up and you can't attend grad school at all but it's not good if they find out you have been using their deferral time and taking up a spot they've saved for you just so you can apply to different schools!
In addition, if the schools you apply to next year learn about this deferral you have from this school, they will not be very happy about it either. You may or may not have to disclose this in applications.
In short, don't do it, it's a jerk thing to do. If you are not sure you want to attend that school, you should be honest with them and instead of just asking for a deferral, tell them that you want to also apply to other schools and ask if they are willing to hold their offer for a year. If they say yes, then everything is fine. If they say no, or that you have to reapply, then do that. Having to re-apply is pretty common and a deferral is usually an excuse from having to reapply for some valid reason!
I think it's important for graduate students like us to remain professional and take personal/professional responsibility for the consequences of our actions.