Thailand is the only country in the world who have a 90 days report policy. (I did a google search)
I believe it's made to get more money from foreigners who forget to report, especially older people with alzheimer or bad memory.
If a foreigner staying in the Kingdom over 90 days without notifying the immigration bureau or notifying the immigration bureau later than the set period, a fine of 2,000 Baht will be collected. If a foreigner who did not make the notification of staying over 90 days is arrested, he will be fined 4,000.