Intent matters, but so does functionality and both have the same base functionality. Abusing them to share large files like MegaUpload would be a bit more work, but not much.
You're mincing words for the sake of being argumentative. The services provided by Megaupload type sites and the services provided by dropbox, skydrive, et al. are fundamentally different and you know it, right down to the functionality.
The advertisements and marketing, the clickthrough monetization, the method of uploading and downloading, the intent, all of it is different.
The cloud sync services are based around your account, everything you do is directly tied to your user account. You log onto each device you want to sync with the same user account, and the purpose of the service is primarily that synchronization between devices. You can subscribe and pay the provider for things like more available space, and you dont get paid if someone downloads your links.
The filesharing services are essentially a pretty web interface for a seedy open FTP server. You dont need an account to upload and share your file, you literally go to their site, select the file, and it spits out a link to share with other people. Some of them have very minimal account based features like folders. They make their money off two things: locking download speeds behind premium subscriptions, and clickthrough traffic based ad revenue. The latter, they generate by saying "hey users, make an account and share files with people. For every 100 downloads we'll give you a penny!" So they pay out a penny after making a few dollars in ad revenue from those 100 pageviews, and get the users to do all the legwork.
These schemes *clearly* are not designed with sharing some family photos with your friend in mind, as that doesnt generate nearly enough hits to make the user any money. They're designed to encourage you to upload things LOTS of people want (warez, porn, movies/music/etc), and plaster the download link all over every warez forum on the internet. You make money, they make money, and nobody gives half a shit about the law because the company hides behind the protections of their neutrality in only taking down content that is reported as infringing.
Just because both services allow someone to generate a download link does not make them the same thing by any means.