You do have a point though... but I'm sure I played exactly the same demo of PvZ before the game even came out on Xbox Live, so it's not really much of a benefit. (It might be a completely different demo, I obviously haven't had time to play it, and likely
Maybe the other games are more worth having a PC version, and if they extend it to new releases so you could play on the PC and Xbox then great.
Microsoft have been going on and on recently about how the future is HTML5. If the reports about the new dashboard are true, it will have a HTML5 front end. There are also lots of rumours about Microsoft investigating free to play games.
Now, if these games are HTML5, then its quite possible this is a test bed, for the next dashboard/WP7 Mango/Windows8.