Would you really trust the temp checkers to keep themselves and you free from the virus the guy in front of you has?
Yes. Stores, hospitals, restaurants and companies have a vested interest in making this work.
A way around your concern - which is valid - would be to require two checkers 10 feet apart... An additional requirement for restaurants is half their tables need to be removed or left empty and all people in a group would be rejected if one person in the group tests positive. Rejected groups are allowed to order takeout...