Yeah, unfortunately the beastmaster's animal is less of a 'companion' right now, and more just another combat feature. I have criticized this in the past because the ranger's animal companion should be, as you say, an animal I choose once and then keep until it dies. Currently it is an ability with which you can summon an animal out of a select few, and unsommon whenever you like as well as change your choice of animal. Ultimately, this doesn't feel like the beastmaster.
So I think you're right to complain, this needs to be changed, otherwise the class just feels like an animal summoner.
