There is no law against holding two jobs, but the same business isn't going to employ the same person twice in two different positions.

There is no law saying a customer can only buy one item, but businesses limit some items to one person customer during promotional events.

There is no law against using multiple coupons or discounts, but businesses will only let you apply one discount at a time.

At the end of the day, bounty campaigns operate like a business - you are employed by them to provide a service, and they pay you for your time (even if the payment is some utterly worthless token). They are free to stipulate any rules they wish, regardless of what the forum rules say. If the bounty rules clearly state "no alt accounts", and you willingly ignore/break that rule, then you are liable to receive red trust for dishonesty. If you don't agree with this rule, then you are free not to work for these companies/projects.

The forum rules also allow local rules to be implemented on top of forum rules, and allow self-moderated threads where posts can be deleted by the topic starter for any reason they so wish, regardless of whether or not these posts break forum rules.