There are many reasons that couples might choose to share a facebook account. They aren't all trust issues. Perhaps one partner in the couple simply isn't computer savvy. They want to stay in touch with family and friends and be in the loop, but they can barely navigate their cell phone, let alone facebook. In cases like this it's totally understandable that they would choose to share. It's not to be 'cutesy' or anything of the sort, it's simply a matter of convenience and staying in the loop. While it may be inconvenient to friends on the account, it's very convenient for the couple and helpful.
Still others use a shared account as a form of accountability. That's not to say they don't trust one another, but that they are focusing on ways to strengthen their relationship. If there is no question of infidelity because they are sharing their lives, it isn't an issue. They could be couples from backgrounds where accountability wasn't used and the relationship ended due to infidelity, drugs or alchool. While it may scream distrust, it may also be a way of saying, "I want to show my partner that I am not doing those things". Accountability isn't distrust, it's a form of keeping oneself from danger. Healthy boundaries are important in any relationship.
Setting healthy boundaries isn't anything at all about trust issues. It's about being mindful of the portrayal of oneself to others. It's presenting a united front to others in and making a bold clear statement that this is a couple. Unless we know the reason that a couple has joined their two accounts, it's probably not something we have the right to judge. Even if we do know the reason, it's still a personal and perhaps private matter as to why they have joined the accounts. In this day and age, with such things as the Ashley Madison scandal and other infidelities in the news, I think more couples might be considering merging their accounts if for no other reason than presenting a united front. While it may not work for everyone, it definitely works for some.