I'm betting the banks will offer higher rates on their Tax Free Savings Accounts because they aren't meant for day-to-day transactions and, while withdrawals are not subject to tax, that affects your contribution limit to your TFSA until the following year. Therefore, the banks have somewhat of a stable, guaranteed deposit base similar to that of a GIC or Term Deposit since you're not as likely to be pulling from it regularly. Rather, you are more likely to contribute to it over time.
Remember, though, for 2009, everyone's contribution limit is $5,000 regardless of how many TFSAs you have at different institutions. For this reason, if you have an HSBC Direct Tax Free Savings Account, I recommend a regular Direct Savings Account to put monies into over and above the $5000. Further, institutions will not be issuing tax receipts (as there is no tax deduction, you just don't pay tax on capital gains or investment income) so it's your responsibility to keep track of your contributions and withdrawals across all your TFSAs. You will, however, receive your contribution limit on your Notice of Assessment from Canada Revenue Agency each year, like you do for RRSPs.
Hope that helps,
Doug