In addition to all the pedantic smart arses "sucking the life out" of the sign by spotting the grammatical errors, many of the commenters generally think it's accurate. "This is the first 100% accurate MTA advisory I've ever read." said Polite New Yorker. In fact another commenter says the grammar "makes the sign all the more MTA authentic"
Perhaps the Tube should put signs like these up rather than making comments about stopped clocks as their next April Fool. Problem is that most people would take them seriously or at least think - "Well for once they're being honest".