What determines whether something requires wifi vs network internet traffic? Most tasks can be done using the network's internet connection (3g for me) like uploading photo to facebook or downloading a new app. But sometimes I end up having to wait to find a wifi connection to say, upload a video to youtube or download a game for my Xperia Play.  Why do these tasks require wifi?  Are these software restrictions placed by my provider?
Most apps do this now to protect the user's mobile data usage. Knowing that most data plans are limited (200 MB a month, in the case of my roommate), developers prevent users from, say downloading a 45 MB game from the Play Store/Amazon App Store via mobile network, as this would not be beneficial, seeing as it would take almost 1/4 of your data and would take a significant amount of time and battery. Same goes with uploading.

Most apps do this as an option, though some (in the case of the Amazon App Store) will block you completely from performing these actions without Wi-Fi. But no, this is not an issue with your carrier. I have Sprint (and unlimited data) and I am still forced to download certain apps and upload pictures through various services only through Wi-Fi. 

NOTE: I have found that some apps that prevent Wi-Fi-less downloads (i.e. Amazon App Store) allow me to do these on my network as long as it is over 4G. If you have this option, try it and see if it works for you as well!