Meh... because on this issue I don't care? I actually kind of like that Verizon and Motorola are managing the OS on my phone because I want it to just work. I am a little bummed that in 2 years when I want to upgrade the OS and they want me to upgrade the handset I'll have to upgrade the handset... but I'll get over it because it's a $200 device that I just want to work.
If I wanted an Android phone I could put custom OS's on, I would've bought a Nexus One. But I don't.
The thing you have to remember is that it's a balance of features, cost, usability, openness, etc. Virtually every purchase you'll have make has pros and cons, you determine what is precisely important to you and then make your choice. For the people who are buying the DroidX, an open bootloader is not one of them.
The portion that makes it tricky is those who don't know what a bootloader is... fact is they're a much larger market force than you or I. That's when regulators step in and decide what should be done for the sake of the consumer. You can argue that regulators should step in here, signed ROMs only circumvent the consumer's right to own their own property or some such, but I personally think you'd be stretching it. But asking why consumers would ever support a company that does this is willfully ignoring that a) most people don't know what you're talking about and b) many of us who do know what you're talking about just don't care.
My car also has a governor that keeps me from going over 120mph. But I still bought it as well, speeds over 120mph aren't something I'm too concerned with.
And you know, the menus on my TV are locked. Sony didn't give me the options to flash the OS on my TV, very upsetting... still bought it, which open TV did you buy?