I'm going to go against the grain and say you should add your 1/2 level to the attack. Since the addition of magic ammunition in AV2, using only the bonus listed on the stat block effectively makes these items extremely sub-par.
I threw a few examples together, comparing magic ammo to alchemical items. I'm using a ranger as the class using the magical ammo, the class of the alchemical thrower is moot since are no benefits that any base class gains. (Note: I don't play Eberron/Dark Sun so I'm not aware of any classes from those campaigns)
The reason I chose a ranger is because from what I've seen on other posts, there is concern that adding 1/2 level can be abusive to the fact that players would mass purchase level 1 items and spam them. This of course assumes that they can carry all those items (probably, with a bag of holding) and that they would spend their encounters burning minors (to equip) and standards (to throw) rather than actually using any of their at-will / encounter / utility / daily powers. It also assumes at least 1 free hand to keep grabbing all these items.
My point here is not to state whether the rules say you should or shouldn't add the bonus (which I still can't find absolute proof either way given how the alch powers are written). I'm just showing how attacks and damage would play out if you did add 1/2 level, and how that comparably stacks up against another ranged player.
For damage on the arrow, I'm assuming a basic at-will of 1(W), and 2(W) at level 21. Bow damage (Long Bow) = 1d10.
For ammunition, I chose a Freezing Arrow vs. Alchemical Frost since they both do relatively the same effect.
In the end, decide for yourself:
Examples: Level 1 w/lvl 3 ammo
Lvl 1 Ranger
1/2 lvl = 0
Dex 18 = +4 mod
Bow = +2 prof
Lvl 3 Freezing Arrow = +1 atk/dmg, and if you hit do 1d6 cold per plus and is slowed.
Total bonus to hit = +7
Range = Range of Bow
Benefit: Can use the arrows property with bow property AND with an attack power.
cost per arrow = 30g
Examples: Lvl 1 w/lvl 1 alch item
Lvl 1 Alchemist
1/2 lvl = 0
Lvl 1 Alch Frost = +4 atk, and if you hit target takes 1d10 cold and is slowed, miss = half dmg.
Total bonus to hit = +4
Range 5/10
Benefit: No additional benefits
cost per alch item = 20g
WINNER @ Lvl 1:
Attack = Arrow
Damage = Arrow
Range = Arrow
Miss = Alch
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Examples: Level 21 w/lvl 3 magic ammo
Lvl 21 Ranger
1/2 Level = +10
20 Dex = +5
Bow = +2 prof
Lvl 3 Freezing Arrow = +1 atk/dmg, and if you hit do 1d6 cold per plus and is slowed.
Total bonus to hit = +18
Range = 20/40
Benefit: Can use the arrows property with bow property AND with an attack power.
cost per arrow = 30g
Examples: Lvl 21 w/lvl 3 alch item
Lvl 21 Alchemist
1/2 lvl = +10
Lvl 3 Alch Frost = +4 atk, and if you hit target takes 1d10 cold and is slowed, miss = half dmg.
Total bonus to hit = +14
Range 5/10
Benefit: No additional benefits
cost per alch item = 20g
WINNER @ Lvl 21:
Attack = Arrow
Damage = Arrow
Range = Arrow
Miss = Alch
------
Examples: Level 21 w/lvl 18 ammo
Lvl 21 Ranger
1/2 Level = +10
20 Dex = +5
Bow = +2 prof
Lvl 18 Freezing Arrow = +4 atk/dmg, and if you hit do 1d6 cold per plus and is slowed.
Total bonus to hit = +21
Range = 20/40
Benefit: Can use the arrows property with bow property AND with an attack power.
cost per arrow = 3,400g
Examples: Lvl 21 w/lvl 21 Alch Item
Lvl 21 Alchemist
1/2 lvl = +10
Lvl 21 Alch Frost = +24 atk, and if you hit target takes 3d10 cold and is slowed, miss = half dmg.
Total bonus to hit = +34
Range 5/10
Benefit: No additional benefits
cost per Alch Item = 9,000g
WINNER @ Lvl 21:
Attack = Alch
Damage = Arrow (2d10+4d6+4 vs 3d10)
Miss = Alch
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Adding 1/2 level with low level items, the magical ammunition is superior in attack, range, and damage. In addition it can be used in conjunction with attack powers, the crit bonuses (if any) are additive, and you can use weapon powers (but not enhancements)
Adding 1/2 level with appropriate level items, the alchemical items begin to outshine the magical ammunition in terms of attack bonus. However, Ranged items still win in range, and damage. Magical ammunition also still win for additive powers / property effects which stack on the attack power, weapon, and ammunition.
So using low level alchemy items to become a ranged player, you'll never be more powerful or accurate than a true ranged class. At best, you'll be on-par with a caster class using a low level implement, doing less damage, and having to spend money for each attack. At high levels with high level items, alchemy will be pretty damn accurate, and pretty damn expensive. By comparison magic ammo would also be expensive, have less of chance to hit, but have the potential for multiple effects, much improved range, better damage, and the flexibility of using within appropriate attack powers. In my eyes that's a fair trade-off, rules be damned regardless of whether they allow it or not.
This also somewhat fixes alchemical 'ammunition', since you'd trade off higher damage, attack power/property benefits for a greater chance to hit.
In closing, I'll reiterate my opener and say add it in. If you're spending the feat / class power to become an Alchemist, spending money on regeants, crafting and carrying these things, you may as well get your money's worth.