"Why does the fact that apps are locked make you decide not to develop for the platform?"
From a user's perspective, it's a matter of controlling your own device. Take the iPhone, for example. Apple does not like Flash, and therefore they prohibit Adobe from offering Flash Player to iPhone users. The result is that iPhone users are prohibited from watching video content, playing games, or generally using websites that incorporate Flash content in their websites. Likewise, Apple has rejected applications because:
-> The level of shine on a chat bubble was too similar to Apple's own.
-> Browsers, RSS readers, etc. provided users an alternative to Apple's own apps.
-> An eBook reader was capable of letting the user read potentially-offensive eBooks he/she downloaded.
-> A dictionary defined non-obscene slang and 'urban' words.
-> A Twitter client let users view Twitter posts. Reason: There was a rude post on Twitter.
-> A music streaming app let users stream too much music.
-> An app contained a photo of President Obama that was eventually used on a US stamp.
The list goes on. The point: It's Apple's way or the highway. Microsoft is, unfortunately, following in Apple's footsteps on this one, which is one reason I, and many others, have no intention to buy a Windows Phone 7 device.
In addition, developers are no longer allowed to choose how they sell their own apps. With Windows Mobile, for instance, developers could sell the apps on their own websites, through third-party stores, or through the Windows Mobile Marketplace. They could offer a Windows installer or a CAB installation file. They could bundle the desktop client with the mobile app. They could offer a free trial that limited the features and/or usage time in any way they wanted. They could write native apps that offered greater functionality and better performance. They had a choice. With a mandated app store, the developer must give Microsoft/Apple/etc. it's share (typically 30% of all proceeds, plus a monthly/yearly/per-app fee), cannot offer bundled versions, cannot offer Windows-based installation, cannot write native apps, and are limited to what kinds of free trials they offer. (Apple actually prohibits demo versions, forcing developers to offer one full version and one feature-limited version rather than letting the user try the full app for a short time.)
I could go on, but the above should provide sufficient insight into some of the main issues at play. Thankfully, not all phone operating systems are locked down in such ways. Google's Android, Microsoft's Windows Mobile (now obsolete), Palm's WebOS, Symbian Foundation's Symbian, and Intel/Nokia's MeeGo are all open for general development, despite offering their own app stores that users and developers can choose to participate in. Most people may like the convenience of an 'official' app store, but they also want a choice. Android's marketshare speaks volumes to that effect.