Read the fine print
http://www.freenom.com/en/doc_tcfree_freenom_v0110.pdf
By accepting this Agreement, you hereby acknowledge and agree that we, in our sole discretion, may modify or
discontinue, temporarily or permanently, the service at any part thereof, at any time with or without prior notice,
including, without limitation, modification or discontinuance of advertising, content and applications appearing as
part of FREE DOMAIN. You agree that we will not be liable to you or to any third party for any modification,
suspension or discontinuance of FREE DOMAIN or any part thereof.
https://disqus.com/by/disqus_DAHCAbS93L/
While you may be able to "register" a .tk for free at freenom, it's subject to a number of terms and limitations:
1. You do not have any legal rights to your registered domain name (ie. you don't own it). Which means you cannot transfer it.
2. 1, 2, and 3 character domain names cannot be registered for free. Neither can domain names they determine to be "special", which includes things like dictionary words.
3. You must have an existing website that will be served for your domain name. You cannot park any free registered domain name.
4. The content of your website cannot contain certain subject matter. For example, for the .gq/.cf TLD's, your website cannot contain "vulgar statements [or] rudeness", among other things.
https://en.wikipedia.org/wiki/.tk
If a domain violates any of these terms, it is replaced by a Sedo ad page, and no beforehand warning is given.