Disclaimer: IANAL / I'm Thai but not currently living there.
I can't comment about the guest vs the hotel, but this one is also filed under Section 14(1) in Thailand's Computer-Related Crime Act in addition to Defamation by advertising, so it is another chapter in a long and ongoing story of Section 14(1) abuse, which says:
"Section 14: Whoever commits the following offences shall be liable to an imprisonment [...]: (1) Dishonestly or by deception, entering wholly or partially distorted or false computer data into a computer system in a manner likely to cause damage to the general public; which is not a defamation under the Criminal Code" (unofficial translation)
The original intention for this Section was to be used for _phishing and fraud_ (as it was originally based on Budapest Convention), but it is worded so loosely that it applicable to anything deemed "false information" as long as it is not directly a defamation. This is mainly due to the use of word "false information" instead of "forgery" in the Thai version. This Section (and the whole Section 14) has been used by government to suppress any political differences for a long time, and never once actually be used for phishing or fraud as it was intended.
It also doesn't help that this Section 14(1) actually has higher fine/jail time than the relevant Criminal Code and cannot be withdrawn, unlike the Criminal Code. Group of lawyers at iLaw[1] has been pushing to amend this Act for a long time, but lawmaker only respond to them by making it worse (to the point that it cannot actually be used for phishing or fraud[2]).
Right now the push to amend Section 14(1) kinda stalled, as Thailand has much bigger problem that need to be fixed (including amending the constitution so Army-tied party no longer get automatically elected, of which a signature of 100k Thai people with a hard copy of their National ID card recently been dismissed in favor for "100k" names in Google Forms from the pro-government side).
[1]: http://ilaw.or.th/
[2]: http://ilaw.or.th/node/5705 (in Thai)