> 'Hi, can you implement <Feature that takes a month of work>? Thanks!'
and
> 'Can you implement <Feature that takes a month of work>?'
Are very differently loaded questions! The first one very much indicates no concern about the implementers time, and is very often seen coming from people that have no idea the complexity of the work involved with the "Thansk!" indicating the implementer would automatically accept it regardless of their own priority, while the second one is way more innocent and open for a reply back with "not sure if I have time for that atm".
EDIT: In short, it's a matter of etiquette, the same way nobody wants to deal with a complete fucking asshole (sorry, personal scars here...).