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I don't think this is actually a bug, and the return 
value of tag_bind really is useless.  tag_bind(t,s) 
returns the command bound to that sequence.  
Tkinter.py supports a feature where a binding can 
return the string "break" if no further bound function 
should be invoked, so when adding a binding, Python has 
to synthesize the 'if {123456foo ...}' Tcl command, and 
that's what tag_bind returns.

I think your example code should be written as:

bind_id = c.tag_bind(tId, '<Button-1>', foo)

And then refer to bind_id when you need to delete the 
binding.  

Was your example usage of tag_bind() described in some 
documentation, or in a demo script somewhere?  
Maybe that script or documentation needs to be fixed.