I disagree strongly with limiting the ability to paste Google links into comments in any way. There is a category of brain-dead, exceedingly trivial questions, that can be answered by a link to the proper, authoritative page in the respective manual ("How do I replace a string in PHP?"). In those cases - and only those! - informing the OP of the existence of search engines is perfectly okay in my eyes.
Sometimes, it is right to comment "Just Google it" and a (polite) RTFM in the form of a manual link. Not always, mind, and it's always good to err on the lenient side. But sometimes, it really is.
Answers consisting of just a Google link or a mere RTFM are not okay, for the reasons you state. But as @dmckee points out, those are usually downvoted into the ground anyway.
Current example where the user should have searched SO
