Wikimedia-hosted websites are unique in these two respects (among many others): (1) They are guided by a mission to broadly share information (without any expectation that it will be hosted on Wikimedia sites or carry attribution to Wikimedia itself), and (2) They are funded by a budget that is unusually capable of supporting this ambitious goal.
The MediaWiki software, a free/open source software product created by the Wikimedia Foundation and Wikimedia community, has built-in features that actually *encourage* embedding content. Any website based on the MediaWiki software automatically has the ability to embed content from Wikimedia Commons, with simple code, for example:
But it's not only MediaWiki-based sites that may embed media hosted on Wikimedia servers.
Engaging in Disruptive and Illegal Misuse of Facilities * Posting or distributing content that contains any viruses, malware, worms, Trojan horses, malicious code, or other device that could harm our technical infrastructure or system or that of our users; * Engaging in automated uses of the site that are abusive or disruptive of the services and have not been approved by the Wikimedia community; * Disrupting the services by placing an undue burden on a Project website or the networks or servers connected with a Project website; * Disrupting the services by inundating any of the Project websites with communications or other traffic that suggests no serious intent to use the Project website for its stated purpose; * Knowingly accessing, tampering with, or using any of our non-public areas in our computer systems without authorization; and * Probing, scanning, or testing the vulnerability of any of our technical systems or networks unless all the following conditions are met: * such actions do not unduly abuse or disrupt our technical systems or networks; * such actions are not for personal gain (except for credit for your work); * you report any vulnerabilities to MediaWiki developers (or fix it yourself); and * you do not undertake such actions with malicious or destructive intent.