More dangerous, yup, by we aren't deciding anything by a sole criteria. Motorcycles vs seat belts is obvious, people love bikes there's bike culture people belong to bike clubs they bike race, they live for biking. And on the other hand, seat belts. You really should be able to easily see a strong argument differentiating the two, else how will you respond to someone who does try to extend the same logic?
So such things are decided not on a rational, objective review, but by seeing which group has more public support and then restricting the minority group?

Also, you didn't really respond to my point about warrant-less searches. Letting the government bug everyone's phone would let people go about their daily lives while helping to capture the guilty. If you can't decide whether or not to wear a seatbelt, then having some unnoticeable phone tap running while you talk surely won't impede your freedom.

Both cars and motorcycles serve different purposes to society and both are required. Motorcyclists should also be subject to safety laws, like wearing a helmet.
How are motorcycles required? And helmets serve little purpose on a motorcycle, they simply move the injury from the head to the neck.

This ties in nicely with the endangering of others, if the motorcyclist crashes into a truck for example, he will only get himself killed, trucks are also very dangerous vehicles by the way, often not for the driver but for everyone else, if the driver of the truck has to wear a seatbelt he will have to live with what he has done though instead of taking the easy way out (out of the truck and out of life). Since helmets are mandatory here on motorcycles, you could say it comes down to ensuring a relatively high security standard relative to what is possible on the vehicle you're riding.
If a person is not wearing a seatbelt then they only person they're going to harm is themselves. It doesn't make sense to fine people for not having seatbelts but let people ride much more dangerous vehicles.

Saying that seatbelt use decreases injuries, while true, opens the door to allowing government to regulating many aspects of our personal lives for 'our benefit' - like food - in order to reduce government costs.

Almost everything you do that harms yourself, ends up harming another. Unless you are recluse who is forever alone in this world, harm upon yourself drains other people. By not buckling your seat belt and dying because of it, you have created a destructive ripple in the lives of other people. The other people in the accident will think they may have killed a person depending on how the accident goes down. Everyone has parents, parents have children, friends, they all suffer negatively from your actions of not protecting yourself from imminent danger. Your outlook that as long as what you do does not physically harm another person is completely moronic in my opinion. If we applied that same train of thought to all aspects then we need to strike down laws against mental abuse of any form.
And if we apply your train of thought the government ought to be able to greatly regulate what we can and can't do to ensure our safety, since apparently the feelings of others hold greater control over what we are allowed to do then ourselves.

Finally, if seatbelts are so important, why don't US school buses have them?

CR