They make it opt out so they don't need to demonstrate the cost/benefits of making it required.

They make it opt out because there are always a few people who object[1], so this is a safety valve.

If everyone opted out ("I am Spartacus") then it would stop and they would have to switch to less efficient means. (If it weren't less efficient then they wouldn't need this one.)

As John Gilmore points out at http://new.toad.com/gnu/ :

"If you politely decline to show ID whenever someone asks (or demands) it, and continue politely declining regardless of how they escalate, you will discover what your rights are. You'll be surprised. You'll get away with it. Most of the people who were asking for it have no right to demand it. They've been relying on your voluntary cooperation. They forgot to tell you that part; but you just found it out for yourself. Sometimes you may discover that you didn't have the right to live, move around, or do business in your own country without government-issued documents. That's very interesting knowledge to acquire first-hand too. If you haven't recently tried exercising your right to exist and live without government permission, are you sure you still have that right?"

[1] In one of the author Robert Heinlein's biographical accounts he walks out of a hotel because they demand to see id at registration. He went to another hotel which did not.