Duty to inform immigration when a foreigner rent your place ?

In practice, who would bother with this, especially if such foreigner is only staying for one or two nights and then leaves. I have never heard of anyone doing this and since they removed the information about address registration on incoming passengers cards fewer people than in the past even know about this requirement, which I thought was more characteristic of communist nations like Vietnam and China than Thailand. In any case, I have stayed in private accommodation for years and due to leaving the country at least once every 90 days have never reported my address. In the past, I thought it was only for those staying longer than 90 days anyway.

Correct. In the general population almost no one knows about this requirement and even less care.

Exactly. I'm "sure" all the Isaan landlords up here do this religiously. (NOT). Heck, my landlord doesn't even know my telephone number (OK, he's got my address). Technically, according to the law, if you visit a Thai person in their home (over 24 hours), they are required to notify the local police. Applies also, of course, to guest houses, hotels, campgrounds, etc. Yeah, right.

Many laws in Thailand are quite draconian and Big Brother-ish. Actual enforcement is pretty lenient, liberal, and in cases like this mostly non-existent. The last three hotels I stayed in (Khon Kaen, Phitsanulok, Chiang Mai) didn't even have me fill in the guest card nor asked for my passport. I just paid the money and got the key. (No, it wasn't the pay-by-hour place!).

The only troubling thing is, the laws are still on the books for any pissed-off Thai to use as a handle to give grief to a foreigner in a pay-back situation.