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.
The Wikimedia Terms of Use, as rewritten in 2012, have only the following to say on the topic of embedding content. This is pretty unusual, in comparison to other non-profit and for-profit websites' terms:
Quoted from Terms of Use
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.