The US plans for a lot of situations, as does any competent military. Are you trying to say that the US intended to make these strikes all along, rather than in retaliation for strikes against them?
The ships are there to protect trade. Up until now, they have been focused on shooting down missiles and preventing attacks on trade ships. They're also there to bring a bigger gun to protect Israel if anyone wants to jump into war with them. There is an element of bullshit to this, as the US is basically allowing Israel to commit genocide, but at the same time the US is supposed to protect its allies.
Most of the ships that have been targeted by the Houthis were not going to Israeli ports and have no affiliation with Israel. When they have claimed there was some affiliation, it's really obtuse and far fetched - like one guy in a parent company that has no real involvement with the ship or its cargo.
I don't know the specifics of the US' previous blockade or the famine in Yemen. If you can point me to some information on that I'd appreciate it, right now my searches are drowned out in coverage of the latest events. However, the whole conflict in Yemen is a mess of proxy parties fighting on behalf of others, it's a tangled web with no good parties - as usual, the only ones really winning are the people selling weapons.