Power of Attorney - why revocable???

I am in Texas. I am just starting to learn about sub2. I saw an example of one, in which the seller, for $10, issued a very basic deed (not a quitclaim, though), to the owner, along with a very broadly worded power of attorney so that the buyer could do just about anything with the property. At the end, though, there was a paragraph reserving to the seller the right to revoke.

I am puzzled. This right to revoke seems to put the buyer at risk, if he puts money into the house and the seller then revokes.

Can the buyer adequately protect himself by registering the deed as possible? I appreciate any insight.