No, they don't. Attempting the assassination attempt directly by themselves is incredibly dangerous as they could be caught entirely red handed.

By simply waiting for an assassination attempt to come along naturally and then purposefully not protecting him when it does is a way they can take him out without being held directly responsible.

This really can't be called a "conspiracy theory" as if it's in the same league as UFO's. It's the most likely explanation. All other explanations are completely absurd:

1. The professionally trained secret service were outsmarted by a 20 year old with no training whatsoever. They didn't spot him as he took the most blatantly obvious shooting position and only spotted him minutes after several members of the crowd did. Absurd.

2. Trump planned the whole thing as a stunt to boost his campaign. He planned for the bullet to come within a millimeter of his life and graze his ear. Having him turn his head at the exact moment as the shooter takes the shot to graze his ear and not explode his entire head. All as a stunt to gain popularity. Completely absurd.

However...

3. The secret service spotted the assassin but had orders from above to allow the shooting to take place. And so allowed it to happen. Very much possible.

Why? Because Trump is a threat to the mainstream establishment and taking him out means those currently in power can hold on to their power.

This kind of thing happens all the time in other countries. In Russia, if a presidential candidate starts rivaling Putin, they're poisoned or imprisoned. Why do you think that the USA is any different?

In countries around the world, when somebody becomes a threat to the centre power within that country, they are often assassinated.

People simply cannot imagine that the same could be true with the USA. But the USA has become a highly corrupt country, and so it's very much possible.