Originally Posted by A.Ertbjerg
Because his action is ambiguous. 1 chip wouldn't be enough to call so if he wants to call he would put in the same amount.
The problem is that 1 chip isn't enough to call.
This is a call unless he states raise and it is so for the same reason as the oversized chip rule. In order to avoid confusion you have to declare raise otherwise it's a call.
$25 chips play on 1-2nl tables. If he'd put out a $25 chip without saying anything, it'd be a call. Here, he's put out multiple chips, and the value he put out is enough to represent the call plus at least half of the minimum raise, therefore it's a minimum raise. A card room would have to have a specific rule that raises must be announced verbally, to justify ruling the way you think they should.
Edit: actually now that I think about it some more, A.Ertbjerg is right in that this is usually ruled a call. I've seen this scenario play out countless times at Foxwoods: someone raises to $7 (standard raise for TAG regs in the room) and someone behind puts in 2 red chips. This is always understood to be a call, even though it represents the original bet plus at least half of a minimum raise.