1) COMCAST doesn't necessarily know it's a massive overpayment until it's applying the funds, and then, why should they care if someone wants to pre-pay 8.5 years of service? They certainly don't know how old their customers are (well, maybe they do, but it's not expected).
They should have a better system for dealing with these situations, but refusing to take certain sized payments leads to all sorts of issues - I'm thinking of the gas pumps that stop at $75 around where I live - probably made sense back when gas was $2 a gallon, but now many pickup trucks *can't fill up their tank* on one pass. I.e. - what seems like a large dollar amount today may not be one in 10 years, and these sorts of settings often get forgotten. Also, it would require having totally separate business / residential systems for check acceptance - maybe they do now, but if not, it doesn't make a lot of sense to spend that sort of money on a 0.001% of the time occurrence prevention system.
2) Why not? He has to keep it somewhere... unless you're suggesting his mattress. Or railing against Online Banking, but really - he may not be that mobile, so Online Banking is a lot better than having to truck to some local bank, perhaps with far less useful benefits or account rules than some online banks. If you're saying he shouldn't have $6k in an electronically accessible account - that just seems Luddite to me... all banks do ACH that I know of.
Opera 11.1; Windows XP Pro SP3;Intel C2Q6600; 3GB DDR2 1066; 1M/128k DSL; Comodo Internet Security 5.3;Proxomitron 4.5j Sidki 2009-06-06,GPG ID:0x0A1C6EE3