Is the Sims resource legal?

No, in fact, it is not.

Concerning the Sims 3, at the very least, it is stated in their End User Licence Agreement, in section 2, subsection B, paragraph 4, that:

"In exchange for the right to use content contributed by other users through the Software, when you contribute content through the Software, you expressly grant to other users of the Software the non‐exclusive, perpetual, transferable, worldwide, irrevocable right to access and use, copy, modify, display, perform, and create and distribute derivative works from, your contributed content in connection with the Software, and to distribute and otherwise communicate your contributed content as a component of works that they create using the Software, for example, The Sims lots or The Sims videos, without further notice, attribution or compensation to you. You hereby waive any moral rights of paternity, publication, reputation, or attribution under applicable law with respect to EA's and other players' use and enjoyment of such content contributions in connection with the Software." [Emphasis mine.]

As The Sims Resource charges a subscription (as of this writing) to access some of their content, they are in violation of the EULA and thus infringing on EA's intellectual property rights.

So EA legal finally got their act together for Sims 3...

For Sims 1 and Sims 2, the EULA was not that specific. Basically, you respected the creators enough to honor their wishes regarding their creations (pay or free) or you didn't. That's the core of copyright law.