Horses don't need affection as we know it but more they need to be treated kindly and with respect. Of course, those of us bitten by the terminal horse bug will show affection but a scratch on the neck or a nice face rub is probably appreciated more by the horse than a kiss on the nose :) After all, that is how momma horse shows her "affection" to her baby...by nuzzling and rubbing. Has been pointed out, horses do learn to recognize people and they have their favorites :) One horse I leased about 15 years or so back had been a well loved mare for a few years before her young teenage owner got interested in boys; the mare was neglected in that manner though the general maintenance for full board left a lot to be desired. When I found this mare she was 200 pounds underweight and the only word I can call it is depressed..she had no interest in people or the goings on in the barn. A month of daily visits had her perking her ears and calling when she heard my car pull up; it took three months to get the weight back on her (self-care barn so she was on MY feeding program :) ). When her "owner" visited, which was only three times in the year I had her, the mare would tolerate the girl but there was no bond.
People who do what you are describing, just hop on like they would on a motorbike, don't have and will never have, the bond that can develop between a horse and rider..they ride..but they aren't "riders". That is up to the club/barn management to take them aside and let them know about some basic etiquette when it comes to the horses.
At the barn I ride at, this is paramount. Potential boarders are interviewed and screened to make sure they will be a positive addition to the barn...not everyone that has requested to board has been accepted, even if there a bunch of stalls open. Some of it is financial related, past issues with paying board, however, in many cases, it was attitude and general lack of respect that denied them a stall. All of the boarders and people who take lessons know what is expected in regards to caring for the horses on property and the manager and staff are all very careful to keep an eye on the school horses to make sure they are comfortable and healthy. If there is any doubt, any horse at any point in time may be removed from lessons until such a time as they are vet cleared to return to work. Several of the school horses are in their early to mid-20s and are only used in leadline classes.
One major thing I do when moving barns (I was active military so did quite a bit of moving) is I walk into the barns and take a deep breath. It may sound strange but if my eyes start to water from the reek of poorly cleaned stalls, I walk back out and don't return. This is the most basic of care. If the barn staff/manager won't clean the stalls properly, chances are the horses are not being looked after properly either. Caveat..there is a difference between the normal stall cleanliness when horses are in and out. Stalls cleaned regularly, ie daily, may have manure or have a wet spot but they don't reek. What I am referring to are stalls that are cleaned, maybe, every few days and horses are standing in wet and/or manure piled stalls.