Like others has said that time isn't free, and believe it or not most of the time the job is going to take more than an hour or two.
The techs are just service guy; they have other customers with work orders to fulfill, and bosses to answers to. The techs also need to have the work order/bill agreement for them to start the work, because the customer can later chose not pay for the unauthorized worked after they went out of their way to help the potential customer. One more thing, a simple problem at first look could lead up to other unforeseen problems that the AC unit have therefor it could take the tech a few days to repair (could lead up to expensive part/s replacement), and that is a huge risk that no one would want to take with out a work order.
Most service guys would do extra work or something nice to a customer that treat them well, and good luck getting them back if your are rotten to them.
PS. Yup! the service guys that get pay by the hour really going to rip you off when they are running off their feet to keep their only customer happy. I'm sure that there are no other commitment/s that the service guy/s have during a heat wave.
<-- not a service guy, but I have seen how hard the service guys in my shop worked when there is a fluctuation in temperature.