Also, bodybuilding is a totally separate sport to powerlifting.
I’m not sure what you were getting at here so correct if I’m reading too much into what you stated, but the two sports are very similar. They both focus on progressive overload (for bodybuilding at least as a beginner and intermediate), they obviously both use very similar forms of resistance (albeit exercise selection may vary depending on the lifter), diet is integral to success in both sports, rep ranges and volume is also similar the majority of the time for each sport (powerlifters do ‘peak’ their lifts come competition, ie. Handle loads closer to maximal effort. Also, programming in powerlifting does vary a lot so some lifters will focus there entire regime around heavy lifting). You are correct in the essence of powerlifting is to move maximal weight and that of bodybuilding is to create an aesthetically pleasing physique, but this does not mean that they are totally separate sports. Simply put, in general it would not be obvious to someone in the gym as to whether or not someone in training for bodybuilding or powerlifting or both. Finally, many of the great legends in bodybuilding started out in powerlifting and vice versa. I personally would recommend a strength based program to a beginner bodybuilder as to allow them to create a strong foundation in to which to build their body from.
Bodybuilders don't care how much you bench, curl, squat or deadlift.
Powerlifters don’t focus on the curl either, they focus on the other lifts because they are an expression of basic human movements common to everyday life, which are an atleast somewhat accurate representation of a lifters strength. This is just an inherent difference between the two sports, not necessarily good or bad.
There is a reason that 'powerlifting form' exists. It allows you to cheat out of a few kilos on your lifts, so yes, you can lift more weight.
Powerlifting form does not exist to cheat out a few kilos. I don’t know how you have even come to this conclusion. First of all, powerlifters lift in a way such that the heaviest load can be moved most efficiently and safely. I’ll take the bench press as an example, as the form of the squat and deadlift between both sports is identical. The powerlifting bench press simply put encourages scapula retraction, a natural arch in the back and feet to be planted firmly on the floor. The scapulae are retracted so that the shoulder is put into a safer position to move a load to and from the chest. The natural arch means that the scapulae can be more effectively retracted and encourages the lifter to flare the latissimus dorsi and create a stable base to push the weight from, the feet are planted in order to encourage this stable base in which total body tightness can be achieved (again allowing the lifter to safely perform a lift as maximal effort). Secondly, whereas bodybuilders are competing on physique, powerlifters are competing on lifts, again just an inherent difference neither good nor bad. This means that cheating a few kilos will get the lifter’s lift disqualified, (ie. Lifting your glutes of the bench during the bench press, hitching the deadlift by resting on your knees or not hitting an appropriate depth on the squat). The rules on which they base disqualified lifts are to my knowledge put in place for two reasons, one to create consistency as to what is considered an acceptable lift and secondly so that lifters can lift in the safest manner (ie. Squatting to a depth too high puts excessive strain on the knee joint, hitching a deadlift puts excessive strain on the lumbar spine).
It's counterproductive for bodybuilding.
Again, this just isn’t true. As I said before I would actually recommend a bodybuilder to start on an appropriately chosen strength based program. Take a look a Layne Norton’s PHAT program that is precisely a mix of bodybuilding and powerlifting. Layne Norton is a very successful natural bodybuilder who sees how the two sports are very complimentary.