Hm, I always thought the license plate number was so they know not to tow your car when it's left there for a(n otherwise) suspiciously long time.

IME, when filling out the form, the clerks usually just say "oh don't worry about that". So one time, expecting the same, I asked them, "and do you actually need the plate?" and she said "well how long are you staying?" and I said "A few weeks" and she laughed and said "oh then of course I need that, don't want to tow you by accident."

Your theory IMHO doesn't make sense -- if it's for ratting you out and they don't otherwise use it, then this fugitive-on-the-run can just give a fake number.

Their privacy policy, in this case, literally states they may disclose your information with law enforcement agencies.

    Compliance with Law

    This is a United States Site and is subject to the laws of the United States. 
    We may disclose Guest Information to law enforcement agencies, or may be
    required to disclose it during the discovery process in litigation, pursuant to a 
    court order, or in compliance with any applicable law, regulation, rule or ordinance.

https://www.motel6.com/en/faq.html

At least in my country, Spain, guest data must be sent to the police database within 24h of his/her check-in.

Not just Spain. If you’ve ever traveled you’ll know that the first question you’re asked at a hotel upon checkin is “Can I see your passport?”

I knew Estonia did, so I used that as a basis for a search. Apparently they requested basically an RFC from other EU countries on whether they do [1].

Of the respondents, the following require registration: Belgium, Estonia, Finland, France, Germany, Hungary, Latvia, Lithuania, Luxembourg, Portugal, Spain, Sweden, and Norway.

That’s non-exhaustive, I know from other sources that at least within the EU Italy does as well, not to mention countless other countries around the world.

1: http://emn.ee/wp-content/uploads/2016/02/466_emn_ad-hoc_quer...

I can confirm that it is also the case in Italy and Mexico. My family and I stayed there recently. In both countries, they took copies of our passports to send them to law enforcement agencies as part of the hotel check-in process.

This is common in many countries; whether official or not.

I can’t remember the last time I was required to give a license plate number at a hotel. I have on some occasions years ago, but not any time recently. I assumed it was just a matter of parking enforcement. Surely it can’t be a legal requirement if it’s so inconsistent.

I've only stayed at a motel once, and they asked for my license plate (paid with a credit card). I've been to hundreds of hotels and they have never asked.

Were you paying by cash or by card?

When you pay by cash, you need to do that, as well as supply photo ID.

Oh, it’s only if you pay cash? What do they do if you say you don’t have a car?

If you're not in one of five metro areas where you can reasonably navigate without a car, they'll know you're a liar and a scoundrel, and will do nothing about it.

I'm curious what those 5 metro areas are. (NYC is one, obviously.)

My guesses: Chicago, Boston, SF, ??

I worked as a night auditor in a couple of hotels while in college, and I’ve never even heard a rumor of what you’re talking about. We never gave a scrap of information to a LEO unless it was directly related to a crime under active investigation (as in “we chased someone into room 204 and arrested them. Do you have their registration info?”).

As others have said, license plates were purely so that we didn’t accidentally tow guests’ vehicles and we could call someone who was blocking the fire lane.

Sort of pointless since a fugitive can easily lie about their plate.