i'd rather keep my Wifi and bandwidth to myself. better speed for me.
I'm not familiar with FON, but I've seen similar proposals in the US. Typically, there's some kind of QoS going on in the router, so your traffic will have priority over everybody else. Your usage should not be impacted by others - if there isn't enough bandwidth to go around, they'll see a shortage, not you.
there would need to be a good reason to share for me to change my mind.
The main reason for something like this is roaming. If the system is widespread, then you should be able to find an access point when traveling. You don't have to pay money for this access if you're sharing your excess bandwidth with other customers.
What if someone uses your connection to hack into the Pentagon, or share child porn, or something like that ?
I guess you would get all the blame, right ?
It's definitely a possibility. They could set up proper auditing and monitoring in their routers, and their back-end, so connections can be traced to the actual customer account, but this assumes they actually have this set up. And it may be up to you and your lawyer to pull the records for whatever law enforcement agency decides to investigate.
Certainly with cable companies in North America, one generally signs something to the effect of
"this cable is only good for x number of people, otherwise you pay more," but not everyone gets Internet from the cable company.
In the UK, it's far worse given that they have that ridiculous "law" (of recent notoriety), that makes it illegal to
"dishonestly (share) access the internet."
In the US, it will depend on your contract. If it prohibits bandwidth sharing or reselling then you're SOL.
My gut feeling is that cable systems will definitely prohibit this, since bandwidth is shared between all customers on a segment of cable. It may or may not be prohibited for a point-to-point technology like DSL or FiOS, where your bandwidth is capped to an amount you're explicitly paying for.
As for the UK, I don't know the law but if it prohibits "dishonestly accessing the internet", that sounds more like a prohibition against
piggybacking. It doesn't sound like a prohibition against connecting to a third-party access point if you have permission, nor does it sound like a prohibition against giving access permission to the world at large. The key word here is "dishonestly".
(I'm assuming you inserted the word "share" as a form of commentary. If the law explicitly prohibits sharing access, that's something else, but I would be surprised if that is true.)